diff options
Diffstat (limited to 'Documentation/block/biodoc.txt')
| -rw-r--r-- | Documentation/block/biodoc.txt | 1077 |
1 files changed, 0 insertions, 1077 deletions
diff --git a/Documentation/block/biodoc.txt b/Documentation/block/biodoc.txt deleted file mode 100644 index ac18b488cb5e..000000000000 --- a/Documentation/block/biodoc.txt +++ /dev/null @@ -1,1077 +0,0 @@ - Notes on the Generic Block Layer Rewrite in Linux 2.5 - ===================================================== - -Notes Written on Jan 15, 2002: - Jens Axboe <jens.axboe@oracle.com> - Suparna Bhattacharya <suparna@in.ibm.com> - -Last Updated May 2, 2002 -September 2003: Updated I/O Scheduler portions - Nick Piggin <npiggin@kernel.dk> - -Introduction: - -These are some notes describing some aspects of the 2.5 block layer in the -context of the bio rewrite. The idea is to bring out some of the key -changes and a glimpse of the rationale behind those changes. - -Please mail corrections & suggestions to suparna@in.ibm.com. - -Credits: ---------- - -2.5 bio rewrite: - Jens Axboe <jens.axboe@oracle.com> - -Many aspects of the generic block layer redesign were driven by and evolved -over discussions, prior patches and the collective experience of several -people. See sections 8 and 9 for a list of some related references. - -The following people helped with review comments and inputs for this -document: - Christoph Hellwig <hch@infradead.org> - Arjan van de Ven <arjanv@redhat.com> - Randy Dunlap <rdunlap@xenotime.net> - Andre Hedrick <andre@linux-ide.org> - -The following people helped with fixes/contributions to the bio patches -while it was still work-in-progress: - David S. Miller <davem@redhat.com> - - -Description of Contents: ------------------------- - -1. Scope for tuning of logic to various needs - 1.1 Tuning based on device or low level driver capabilities - - Per-queue parameters - - Highmem I/O support - - I/O scheduler modularization - 1.2 Tuning based on high level requirements/capabilities - 1.2.1 Request Priority/Latency - 1.3 Direct access/bypass to lower layers for diagnostics and special - device operations - 1.3.1 Pre-built commands -2. New flexible and generic but minimalist i/o structure or descriptor - (instead of using buffer heads at the i/o layer) - 2.1 Requirements/Goals addressed - 2.2 The bio struct in detail (multi-page io unit) - 2.3 Changes in the request structure -3. Using bios - 3.1 Setup/teardown (allocation, splitting) - 3.2 Generic bio helper routines - 3.2.1 Traversing segments and completion units in a request - 3.2.2 Setting up DMA scatterlists - 3.2.3 I/O completion - 3.2.4 Implications for drivers that do not interpret bios (don't handle - multiple segments) - 3.3 I/O submission -4. The I/O scheduler -5. Scalability related changes - 5.1 Granular locking: Removal of io_request_lock - 5.2 Prepare for transition to 64 bit sector_t -6. Other Changes/Implications - 6.1 Partition re-mapping handled by the generic block layer -7. A few tips on migration of older drivers -8. A list of prior/related/impacted patches/ideas -9. Other References/Discussion Threads - ---------------------------------------------------------------------------- - -Bio Notes --------- - -Let us discuss the changes in the context of how some overall goals for the -block layer are addressed. - -1. Scope for tuning the generic logic to satisfy various requirements - -The block layer design supports adaptable abstractions to handle common -processing with the ability to tune the logic to an appropriate extent -depending on the nature of the device and the requirements of the caller. -One of the objectives of the rewrite was to increase the degree of tunability -and to enable higher level code to utilize underlying device/driver -capabilities to the maximum extent for better i/o performance. This is -important especially in the light of ever improving hardware capabilities -and application/middleware software designed to take advantage of these -capabilities. - -1.1 Tuning based on low level device / driver capabilities - -Sophisticated devices with large built-in caches, intelligent i/o scheduling -optimizations, high memory DMA support, etc may find some of the -generic processing an overhead, while for less capable devices the -generic functionality is essential for performance or correctness reasons. -Knowledge of some of the capabilities or parameters of the device should be -used at the generic block layer to take the right decisions on -behalf of the driver. - -How is this achieved ? - -Tuning at a per-queue level: - -i. Per-queue limits/values exported to the generic layer by the driver - -Various parameters that the generic i/o scheduler logic uses are set at -a per-queue level (e.g maximum request size, maximum number of segments in -a scatter-gather list, logical block size) - -Some parameters that were earlier available as global arrays indexed by -major/minor are now directly associated with the queue. Some of these may -move into the block device structure in the future. Some characteristics -have been incorporated into a queue flags field rather than separate fields -in themselves. There are blk_queue_xxx functions to set the parameters, -rather than update the fields directly - -Some new queue property settings: - - blk_queue_bounce_limit(q, u64 dma_address) - Enable I/O to highmem pages, dma_address being the - limit. No highmem default. - - blk_queue_max_sectors(q, max_sectors) - Sets two variables that limit the size of the request. - - - The request queue's max_sectors, which is a soft size in - units of 512 byte sectors, and could be dynamically varied - by the core kernel. - - - The request queue's max_hw_sectors, which is a hard limit - and reflects the maximum size request a driver can handle - in units of 512 byte sectors. - - The default for both max_sectors and max_hw_sectors is - 255. The upper limit of max_sectors is 1024. - - blk_queue_max_phys_segments(q, max_segments) - Maximum physical segments you can handle in a request. 128 - default (driver limit). (See 3.2.2) - - blk_queue_max_hw_segments(q, max_segments) - Maximum dma segments the hardware can handle in a request. 128 - default (host adapter limit, after dma remapping). - (See 3.2.2) - - blk_queue_max_segment_size(q, max_seg_size) - Maximum size of a clustered segment, 64kB default. - - blk_queue_logical_block_size(q, logical_block_size) - Lowest possible sector size that the hardware can operate - on, 512 bytes default. - -New queue flags: - - QUEUE_FLAG_CLUSTER (see 3.2.2) - QUEUE_FLAG_QUEUED (see 3.2.4) - - -ii. High-mem i/o capabilities are now considered the default - -The generic bounce buffer logic, present in 2.4, where the block layer would -by default copyin/out i/o requests on high-memory buffers to low-memory buffers -assuming that the driver wouldn't be able to handle it directly, has been -changed in 2.5. The bounce logic is now applied only for memory ranges -for which the device cannot handle i/o. A driver can specify this by -setting the queue bounce limit for the request queue for the device -(blk_queue_bounce_limit()). This avoids the inefficiencies of the copyin/out -where a device is capable of handling high memory i/o. - -In order to enable high-memory i/o where the device is capable of supporting -it, the pci dma mapping routines and associated data structures have now been -modified to accomplish a direct page -> bus translation, without requiring -a virtual address mapping (unlike the earlier scheme of virtual address --> bus translation). So this works uniformly for high-memory pages (which -do not have a corresponding kernel virtual address space mapping) and -low-memory pages. - -Note: Please refer to Documentation/DMA-API-HOWTO.txt for a discussion -on PCI high mem DMA aspects and mapping of scatter gather lists, and support -for 64 bit PCI. - -Special handling is required only for cases where i/o needs to happen on -pages at physical memory addresses beyond what the device can support. In these -cases, a bounce bio representing a buffer from the supported memory range -is used for performing the i/o with copyin/copyout as needed depending on -the type of the operation. For example, in case of a read operation, the -data read has to be copied to the original buffer on i/o completion, so a -callback routine is set up to do this, while for write, the data is copied -from the original buffer to the bounce buffer prior to issuing the -operation. Since an original buffer may be in a high memory area that's not -mapped in kernel virtual addr, a kmap operation may be required for -performing the copy, and special care may be needed in the completion path -as it may not be in irq context. Special care is also required (by way of -GFP flags) when allocating bounce buffers, to avoid certain highmem -deadlock possibilities. - -It is also possible that a bounce buffer may be allocated from high-memory -area that's not mapped in kernel virtual addr, but within the range that the -device can use directly; so the bounce page may need to be kmapped during -copy operations. [Note: This does not hold in the current implementation, -though] - -There are some situations when pages from high memory may need to -be kmapped, even if bounce buffers are not necessary. For example a device -may need to abort DMA operations and revert to PIO for the transfer, in -which case a virtual mapping of the page is required. For SCSI it is also -done in some scenarios where the low level driver cannot be trusted to -handle a single sg entry correctly. The driver is expected to perform the -kmaps as needed on such occasions as appropriate. A driver could also use -the blk_queue_bounce() routine on its own to bounce highmem i/o to low -memory for specific requests if so desired. - -iii. The i/o scheduler algorithm itself can be replaced/set as appropriate - -As in 2.4, it is possible to plugin a brand new i/o scheduler for a particular -queue or pick from (copy) existing generic schedulers and replace/override -certain portions of it. The 2.5 rewrite provides improved modularization -of the i/o scheduler. There are more pluggable callbacks, e.g for init, -add request, extract request, which makes it possible to abstract specific -i/o scheduling algorithm aspects and details outside of the generic loop. -It also makes it possible to completely hide the implementation details of -the i/o scheduler from block drivers. - -I/O scheduler wrappers are to be used instead of accessing the queue directly. -See section 4. The I/O scheduler for details. - -1.2 Tuning Based on High level code capabilities - -i. Application capabilities for raw i/o - -This comes from some of the high-performance database/middleware -requirements where an application prefers to make its own i/o scheduling -decisions based on an understanding of the access patterns and i/o -characteristics - -ii. High performance filesystems or other higher level kernel code's -capabilities - -Kernel components like filesystems could also take their own i/o scheduling -decisions for optimizing performance. Journalling filesystems may need -some control over i/o ordering. - -What kind of support exists at the generic block layer for this ? - -The flags and rw fields in the bio structure can be used for some tuning -from above e.g indicating that an i/o is just a readahead request, or priority -settings (currently unused). As far as user applications are concerned they -would need an additional mechanism either via open flags or ioctls, or some -other upper level mechanism to communicate such settings to block. - -1.2.1 Request Priority/Latency - -Todo/Under discussion: -Arjan's proposed request priority scheme allows higher levels some broad - control (high/med/low) over the priority of an i/o request vs other pending - requests in the queue. For example it allows reads for bringing in an - executable page on demand to be given a higher priority over pending write - requests which haven't aged too much on the queue. Potentially this priority - could even be exposed to applications in some manner, providing higher level - tunability. Time based aging avoids starvation of lower priority - requests. Some bits in the bi_opf flags field in the bio structure are - intended to be used for this priority information. - - -1.3 Direct Access to Low level Device/Driver Capabilities (Bypass mode) - (e.g Diagnostics, Systems Management) - -There are situations where high-level code needs to have direct access to -the low level device capabilities or requires the ability to issue commands -to the device bypassing some of the intermediate i/o layers. -These could, for example, be special control commands issued through ioctl -interfaces, or could be raw read/write commands that stress the drive's -capabilities for certain kinds of fitness tests. Having direct interfaces at -multiple levels without having to pass through upper layers makes -it possible to perform bottom up validation of the i/o path, layer by -layer, starting from the media. - -The normal i/o submission interfaces, e.g submit_bio, could be bypassed -for specially crafted requests which such ioctl or diagnostics -interfaces would typically use, and the elevator add_request routine -can instead be used to directly insert such requests in the queue or preferably -the blk_do_rq routine can be used to place the request on the queue and -wait for completion. Alternatively, sometimes the caller might just -invoke a lower level driver specific interface with the request as a -parameter. - -If the request is a means for passing on special information associated with -the command, then such information is associated with the request->special -field (rather than misuse the request->buffer field which is meant for the -request data buffer's virtual mapping). - -For passing request data, the caller must build up a bio descriptor -representing the concerned memory buffer if the underlying driver interprets -bio segments or uses the block layer end*request* functions for i/o -completion. Alternatively one could directly use the request->buffer field to -specify the virtual address of the buffer, if the driver expects buffer -addresses passed in this way and ignores bio entries for the request type -involved. In the latter case, the driver would modify and manage the -request->buffer, request->sector and request->nr_sectors or -request->current_nr_sectors fields itself rather than using the block layer -end_request or end_that_request_first completion interfaces. -(See 2.3 or Documentation/block/request.txt for a brief explanation of -the request structure fields) - -[TBD: end_that_request_last should be usable even in this case; -Perhaps an end_that_direct_request_first routine could be implemented to make -handling direct requests easier for such drivers; Also for drivers that -expect bios, a helper function could be provided for setting up a bio -corresponding to a data buffer] - -<JENS: I dont understand the above, why is end_that_request_first() not -usable? Or _last for that matter. I must be missing something> -<SUP: What I meant here was that if the request doesn't have a bio, then - end_that_request_first doesn't modify nr_sectors or current_nr_sectors, - and hence can't be used for advancing request state settings on the - completion of partial transfers. The driver has to modify these fields - directly by hand. - This is because end_that_request_first only iterates over the bio list, - and always returns 0 if there are none associated with the request. - _last works OK in this case, and is not a problem, as I mentioned earlier -> - -1.3.1 Pre-built Commands - -A request can be created with a pre-built custom command to be sent directly -to the device. The cmd block in the request structure has room for filling -in the command bytes. (i.e rq->cmd is now 16 bytes in size, and meant for -command pre-building, and the type of the request is now indicated -through rq->flags instead of via rq->cmd) - -The request structure flags can be set up to indicate the type of request -in such cases (REQ_PC: direct packet command passed to driver, REQ_BLOCK_PC: -packet command issued via blk_do_rq, REQ_SPECIAL: special request). - -It can help to pre-build device commands for requests in advance. -Drivers can now specify a request prepare function (q->prep_rq_fn) that the -block layer would invoke to pre-build device commands for a given request, -or perform other preparatory processing for the request. This is routine is -called by elv_next_request(), i.e. typically just before servicing a request. -(The prepare function would not be called for requests that have RQF_DONTPREP -enabled) - -Aside: - Pre-building could possibly even be done early, i.e before placing the - request on the queue, rather than construct the command on the fly in the - driver while servicing the request queue when it may affect latencies in - interrupt context or responsiveness in general. One way to add early - pre-building would be to do it whenever we fail to merge on a request. - Now REQ_NOMERGE is set in the request flags to skip this one in the future, - which means that it will not change before we feed it to the device. So - the pre-builder hook can be invoked there. - - -2. Flexible and generic but minimalist i/o structure/descriptor. - -2.1 Reason for a new structure and requirements addressed - -Prior to 2.5, buffer heads were used as the unit of i/o at the generic block -layer, and the low level request structure was associated with a chain of -buffer heads for a contiguous i/o request. This led to certain inefficiencies -when it came to large i/o requests and readv/writev style operations, as it -forced such requests to be broken up into small chunks before being passed -on to the generic block layer, only to be merged by the i/o scheduler -when the underlying device was capable of handling the i/o in one shot. -Also, using the buffer head as an i/o structure for i/os that didn't originate -from the buffer cache unnecessarily added to the weight of the descriptors -which were generated for each such chunk. - -The following were some of the goals and expectations considered in the -redesign of the block i/o data structure in 2.5. - -i. Should be appropriate as a descriptor for both raw and buffered i/o - - avoid cache related fields which are irrelevant in the direct/page i/o path, - or filesystem block size alignment restrictions which may not be relevant - for raw i/o. -ii. Ability to represent high-memory buffers (which do not have a virtual - address mapping in kernel address space). -iii.Ability to represent large i/os w/o unnecessarily breaking them up (i.e - greater than PAGE_SIZE chunks in one shot) -iv. At the same time, ability to retain independent identity of i/os from - different sources or i/o units requiring individual completion (e.g. for - latency reasons) -v. Ability to represent an i/o involving multiple physical memory segments - (including non-page aligned page fragments, as specified via readv/writev) - without unnecessarily breaking it up, if the underlying device is capable of - handling it. -vi. Preferably should be based on a memory descriptor structure that can be - passed around different types of subsystems or layers, maybe even - networking, without duplication or extra copies of data/descriptor fields - themselves in the process -vii.Ability to handle the possibility of splits/merges as the structure passes - through layered drivers (lvm, md, evms), with minimal overhead. - -The solution was to define a new structure (bio) for the block layer, -instead of using the buffer head structure (bh) directly, the idea being -avoidance of some associated baggage and limitations. The bio structure -is uniformly used for all i/o at the block layer ; it forms a part of the -bh structure for buffered i/o, and in the case of raw/direct i/o kiobufs are -mapped to bio structures. - -2.2 The bio struct - -The bio structure uses a vector representation pointing to an array of tuples -of <page, offset, len> to describe the i/o buffer, and has various other -fields describing i/o parameters and state that needs to be maintained for -performing the i/o. - -Notice that this representation means that a bio has no virtual address -mapping at all (unlike buffer heads). - -struct bio_vec { - struct page *bv_page; - unsigned short bv_len; - unsigned short bv_offset; -}; - -/* - * main unit of I/O for the block layer and lower layers (ie drivers) - */ -struct bio { - struct bio *bi_next; /* request queue link */ - struct block_device *bi_bdev; /* target device */ - unsigned long bi_flags; /* status, command, etc */ - unsigned long bi_opf; /* low bits: r/w, high: priority */ - - unsigned int bi_vcnt; /* how may bio_vec's */ - struct bvec_iter bi_iter; /* current index into bio_vec array */ - - unsigned int bi_size; /* total size in bytes */ - unsigned short bi_phys_segments; /* segments after physaddr coalesce*/ - unsigned short bi_hw_segments; /* segments after DMA remapping */ - unsigned int bi_max; /* max bio_vecs we can hold - used as index into pool */ - struct bio_vec *bi_io_vec; /* the actual vec list */ - bio_end_io_t *bi_end_io; /* bi_end_io (bio) */ - atomic_t bi_cnt; /* pin count: free when it hits zero */ - void *bi_private; -}; - -With this multipage bio design: - -- Large i/os can be sent down in one go using a bio_vec list consisting - of an array of <page, offset, len> fragments (similar to the way fragments - are represented in the zero-copy network code) -- Splitting of an i/o request across multiple devices (as in the case of - lvm or raid) is achieved by cloning the bio (where the clone points to - the same bi_io_vec array, but with the index and size accordingly modified) -- A linked list of bios is used as before for unrelated merges (*) - this - avoids reallocs and makes independent completions easier to handle. -- Code that traverses the req list can find all the segments of a bio - by using rq_for_each_segment. This handles the fact that a request - has multiple bios, each of which can have multiple segments. -- Drivers which can't process a large bio in one shot can use the bi_iter - field to keep track of the next bio_vec entry to process. - (e.g a 1MB bio_vec needs to be handled in max 128kB chunks for IDE) - [TBD: Should preferably also have a bi_voffset and bi_vlen to avoid modifying - bi_offset an len fields] - -(*) unrelated merges -- a request ends up containing two or more bios that - didn't originate from the same place. - -bi_end_io() i/o callback gets called on i/o completion of the entire bio. - -At a lower level, drivers build a scatter gather list from the merged bios. -The scatter gather list is in the form of an array of <page, offset, len> -entries with their corresponding dma address mappings filled in at the -appropriate time. As an optimization, contiguous physical pages can be -covered by a single entry where <page> refers to the first page and <len> -covers the range of pages (up to 16 contiguous pages could be covered this -way). There is a helper routine (blk_rq_map_sg) which drivers can use to build -the sg list. - -Note: Right now the only user of bios with more than one page is ll_rw_kio, -which in turn means that only raw I/O uses it (direct i/o may not work -right now). The intent however is to enable clustering of pages etc to -become possible. The pagebuf abstraction layer from SGI also uses multi-page -bios, but that is currently not included in the stock development kernels. -The same is true of Andrew Morton's work-in-progress multipage bio writeout -and readahead patches. - -2.3 Changes in the Request Structure - -The request structure is the structure that gets passed down to low level -drivers. The block layer make_request function builds up a request structure, -places it on the queue and invokes the drivers request_fn. The driver makes -use of block layer helper routine elv_next_request to pull the next request -off the queue. Control or diagnostic functions might bypass block and directly -invoke underlying driver entry points passing in a specially constructed -request structure. - -Only some relevant fields (mainly those which changed or may be referred -to in some of the discussion here) are listed below, not necessarily in -the order in which they occur in the structure (see include/linux/blkdev.h) -Refer to Documentation/block/request.txt for details about all the request -structure fields and a quick reference about the layers which are -supposed to use or modify those fields. - -struct request { - struct list_head queuelist; /* Not meant to be directly accessed by - the driver. - Used by q->elv_next_request_fn - rq->queue is gone - */ - . - . - unsigned char cmd[16]; /* prebuilt command data block */ - unsigned long flags; /* also includes earlier rq->cmd settings */ - . - . - sector_t sector; /* this field is now of type sector_t instead of int - preparation for 64 bit sectors */ - . - . - - /* Number of scatter-gather DMA addr+len pairs after - * physical address coalescing is performed. - */ - unsigned short nr_phys_segments; - - /* Number of scatter-gather addr+len pairs after - * physical and DMA remapping hardware coalescing is performed. - * This is the number of scatter-gather entries the driver - * will actually have to deal with after DMA mapping is done. - */ - unsigned short nr_hw_segments; - - /* Various sector counts */ - unsigned long nr_sectors; /* no. of sectors left: driver modifiable */ - unsigned long hard_nr_sectors; /* block internal copy of above */ - unsigned int current_nr_sectors; /* no. of sectors left in the - current segment:driver modifiable */ - unsigned long hard_cur_sectors; /* block internal copy of the above */ - . - . - int tag; /* command tag associated with request */ - void *special; /* same as before */ - char *buffer; /* valid only for low memory buffers up to - current_nr_sectors */ - . - . - struct bio *bio, *biotail; /* bio list instead of bh */ - struct request_list *rl; -} - -See the req_ops and req_flag_bits definitions for an explanation of the various -flags available. Some bits are used by the block layer or i/o scheduler. - -The behaviour of the various sector counts are almost the same as before, -except that since we have multi-segment bios, current_nr_sectors refers -to the numbers of sectors in the current segment being processed which could -be one of the many segments in the current bio (i.e i/o completion unit). -The nr_sectors value refers to the total number of sectors in the whole -request that remain to be transferred (no change). The purpose of the -hard_xxx values is for block to remember these counts every time it hands -over the request to the driver. These values are updated by block on -end_that_request_first, i.e. every time the driver completes a part of the -transfer and invokes block end*request helpers to mark this. The -driver should not modify these values. The block layer sets up the -nr_sectors and current_nr_sectors fields (based on the corresponding -hard_xxx values and the number of bytes transferred) and updates it on -every transfer that invokes end_that_request_first. It does the same for the -buffer, bio, bio->bi_iter fields too. - -The buffer field is just a virtual address mapping of the current segment -of the i/o buffer in cases where the buffer resides in low-memory. For high -memory i/o, this field is not valid and must not be used by drivers. - -Code that sets up its own request structures and passes them down to -a driver needs to be careful about interoperation with the block layer helper -functions which the driver uses. (Section 1.3) - -3. Using bios - -3.1 Setup/Teardown - -There are routines for managing the allocation, and reference counting, and -freeing of bios (bio_alloc, bio_get, bio_put). - -This makes use of Ingo Molnar's mempool implementation, which enables -subsystems like bio to maintain their own reserve memory pools for guaranteed -deadlock-free allocations during extreme VM load. For example, the VM -subsystem makes use of the block layer to writeout dirty pages in order to be -able to free up memory space, a case which needs careful handling. The -allocation logic draws from the preallocated emergency reserve in situations -where it cannot allocate through normal means. If the pool is empty and it -can wait, then it would trigger action that would help free up memory or -replenish the pool (without deadlocking) and wait for availability in the pool. -If it is in IRQ context, and hence not in a position to do this, allocation -could fail if the pool is empty. In general mempool always first tries to -perform allocation without having to wait, even if it means digging into the -pool as long it is not less that 50% full. - -On a free, memory is released to the pool or directly freed depending on -the current availability in the pool. The mempool interface lets the -subsystem specify the routines to be used for normal alloc and free. In the -case of bio, these routines make use of the standard slab allocator. - -The caller of bio_alloc is expected to taken certain steps to avoid -deadlocks, e.g. avoid trying to allocate more memory from the pool while -already holding memory obtained from the pool. -[TBD: This is a potential issue, though a rare possibility - in the bounce bio allocation that happens in the current code, since - it ends up allocating a second bio from the same pool while - holding the original bio ] - -Memory allocated from the pool should be released back within a limited -amount of time (in the case of bio, that would be after the i/o is completed). -This ensures that if part of the pool has been used up, some work (in this -case i/o) must already be in progress and memory would be available when it -is over. If allocating from multiple pools in the same code path, the order -or hierarchy of allocation needs to be consistent, just the way one deals -with multiple locks. - -The bio_alloc routine also needs to allocate the bio_vec_list (bvec_alloc()) -for a non-clone bio. There are the 6 pools setup for different size biovecs, -so bio_alloc(gfp_mask, nr_iovecs) will allocate a vec_list of the -given size from these slabs. - -The bio_get() routine may be used to hold an extra reference on a bio prior -to i/o submission, if the bio fields are likely to be accessed after the -i/o is issued (since the bio may otherwise get freed in case i/o completion -happens in the meantime). - -The bio_clone_fast() routine may be used to duplicate a bio, where the clone -shares the bio_vec_list with the original bio (i.e. both point to the -same bio_vec_list). This would typically be used for splitting i/o requests -in lvm or md. - -3.2 Generic bio helper Routines - -3.2.1 Traversing segments and completion units in a request - -The macro rq_for_each_segment() should be used for traversing the bios -in the request list (drivers should avoid directly trying to do it -themselves). Using these helpers should also make it easier to cope -with block changes in the future. - - struct req_iterator iter; - rq_for_each_segment(bio_vec, rq, iter) - /* bio_vec is now current segment */ - -I/O completion callbacks are per-bio rather than per-segment, so drivers -that traverse bio chains on completion need to keep that in mind. Drivers -which don't make a distinction between segments and completion units would -need to be reorganized to support multi-segment bios. - -3.2.2 Setting up DMA scatterlists - -The blk_rq_map_sg() helper routine would be used for setting up scatter -gather lists from a request, so a driver need not do it on its own. - - nr_segments = blk_rq_map_sg(q, rq, scatterlist); - -The helper routine provides a level of abstraction which makes it easier -to modify the internals of request to scatterlist conversion down the line -without breaking drivers. The blk_rq_map_sg routine takes care of several -things like collapsing physically contiguous segments (if QUEUE_FLAG_CLUSTER -is set) and correct segment accounting to avoid exceeding the limits which -the i/o hardware can handle, based on various queue properties. - -- Prevents a clustered segment from crossing a 4GB mem boundary -- Avoids building segments that would exceed the number of physical - memory segments that the driver can handle (phys_segments) and the - number that the underlying hardware can handle at once, accounting for - DMA remapping (hw_segments) (i.e. IOMMU aware limits). - -Routines which the low level driver can use to set up the segment limits: - -blk_queue_max_hw_segments() : Sets an upper limit of the maximum number of -hw data segments in a request (i.e. the maximum number of address/length -pairs the host adapter can actually hand to the device at once) - -blk_queue_max_phys_segments() : Sets an upper limit on the maximum number -of physical data segments in a request (i.e. the largest sized scatter list -a driver could handle) - -3.2.3 I/O completion - -The existing generic block layer helper routines end_request, -end_that_request_first and end_that_request_last can be used for i/o -completion (and setting things up so the rest of the i/o or the next -request can be kicked of) as before. With the introduction of multi-page -bio support, end_that_request_first requires an additional argument indicating -the number of sectors completed. - -3.2.4 Implications for drivers that do not interpret bios (don't handle - multiple segments) - -Drivers that do not interpret bios e.g those which do not handle multiple -segments and do not support i/o into high memory addresses (require bounce -buffers) and expect only virtually mapped buffers, can access the rq->buffer -field. As before the driver should use current_nr_sectors to determine the -size of remaining data in the current segment (that is the maximum it can -transfer in one go unless it interprets segments), and rely on the block layer -end_request, or end_that_request_first/last to take care of all accounting -and transparent mapping of the next bio segment when a segment boundary -is crossed on completion of a transfer. (The end*request* functions should -be used if only if the request has come down from block/bio path, not for -direct access requests which only specify rq->buffer without a valid rq->bio) - -3.3 I/O Submission - -The routine submit_bio() is used to submit a single io. Higher level i/o -routines make use of this: - -(a) Buffered i/o: -The routine submit_bh() invokes submit_bio() on a bio corresponding to the -bh, allocating the bio if required. ll_rw_block() uses submit_bh() as before. - -(b) Kiobuf i/o (for raw/direct i/o): -The ll_rw_kio() routine breaks up the kiobuf into page sized chunks and -maps the array to one or more multi-page bios, issuing submit_bio() to -perform the i/o on each of these. - -The embedded bh array in the kiobuf structure has been removed and no -preallocation of bios is done for kiobufs. [The intent is to remove the -blocks array as well, but it's currently in there to kludge around direct i/o.] -Thus kiobuf allocation has switched back to using kmalloc rather than vmalloc. - -Todo/Observation: - - A single kiobuf structure is assumed to correspond to a contiguous range - of data, so brw_kiovec() invokes ll_rw_kio for each kiobuf in a kiovec. - So right now it wouldn't work for direct i/o on non-contiguous blocks. - This is to be resolved. The eventual direction is to replace kiobuf - by kvec's. - - Badari Pulavarty has a patch to implement direct i/o correctly using - bio and kvec. - - -(c) Page i/o: -Todo/Under discussion: - - Andrew Morton's multi-page bio patches attempt to issue multi-page - writeouts (and reads) from the page cache, by directly building up - large bios for submission completely bypassing the usage of buffer - heads. This work is still in progress. - - Christoph Hellwig had some code that uses bios for page-io (rather than - bh). This isn't included in bio as yet. Christoph was also working on a - design for representing virtual/real extents as an entity and modifying - some of the address space ops interfaces to utilize this abstraction rather - than buffer_heads. (This is somewhat along the lines of the SGI XFS pagebuf - abstraction, but intended to be as lightweight as possible). - -(d) Direct access i/o: -Direct access requests that do not contain bios would be submitted differently -as discussed earlier in section 1.3. - -Aside: - - Kvec i/o: - - Ben LaHaise's aio code uses a slightly different structure instead - of kiobufs, called a kvec_cb. This contains an array of <page, offset, len> - tuples (very much like the networking code), together with a callback function - and data pointer. This is embedded into a brw_cb structure when passed - to brw_kvec_async(). - - Now it should be possible to directly map these kvecs to a bio. Just as while - cloning, in this case rather than PRE_BUILT bio_vecs, we set the bi_io_vec - array pointer to point to the veclet array in kvecs. - - TBD: In order for this to work, some changes are needed in the way multi-page - bios are handled today. The values of the tuples in such a vector passed in - from higher level code should not be modified by the block layer in the course - of its request processing, since that would make it hard for the higher layer - to continue to use the vector descriptor (kvec) after i/o completes. Instead, - all such transient state should either be maintained in the request structure, - and passed on in some way to the endio completion routine. - - -4. The I/O scheduler -I/O scheduler, a.k.a. elevator, is implemented in two layers. Generic dispatch -queue and specific I/O schedulers. Unless stated otherwise, elevator is used -to refer to both parts and I/O scheduler to specific I/O schedulers. - -Block layer implements generic dispatch queue in block/*.c. -The generic dispatch queue is responsible for requeueing, handling non-fs -requests and all other subtleties. - -Specific I/O schedulers are responsible for ordering normal filesystem -requests. They can also choose to delay certain requests to improve -throughput or whatever purpose. As the plural form indicates, there are -multiple I/O schedulers. They can be built as modules but at least one should -be built inside the kernel. Each queue can choose different one and can also -change to another one dynamically. - -A block layer call to the i/o scheduler follows the convention elv_xxx(). This -calls elevator_xxx_fn in the elevator switch (block/elevator.c). Oh, xxx -and xxx might not match exactly, but use your imagination. If an elevator -doesn't implement a function, the switch does nothing or some minimal house -keeping work. - -4.1. I/O scheduler API - -The functions an elevator may implement are: (* are mandatory) -elevator_merge_fn called to query requests for merge with a bio - -elevator_merge_req_fn called when two requests get merged. the one - which gets merged into the other one will be - never seen by I/O scheduler again. IOW, after - being merged, the request is gone. - -elevator_merged_fn called when a request in the scheduler has been - involved in a merge. It is used in the deadline - scheduler for example, to reposition the request - if its sorting order has changed. - -elevator_allow_merge_fn called whenever the block layer determines - that a bio can be merged into an existing - request safely. The io scheduler may still - want to stop a merge at this point if it - results in some sort of conflict internally, - this hook allows it to do that. Note however - that two *requests* can still be merged at later - time. Currently the io scheduler has no way to - prevent that. It can only learn about the fact - from elevator_merge_req_fn callback. - -elevator_dispatch_fn* fills the dispatch queue with ready requests. - I/O schedulers are free to postpone requests by - not filling the dispatch queue unless @force - is non-zero. Once dispatched, I/O schedulers - are not allowed to manipulate the requests - - they belong to generic dispatch queue. - -elevator_add_req_fn* called to add a new request into the scheduler - -elevator_former_req_fn -elevator_latter_req_fn These return the request before or after the - one specified in disk sort order. Used by the - block layer to find merge possibilities. - -elevator_completed_req_fn called when a request is completed. - -elevator_may_queue_fn returns true if the scheduler wants to allow the - current context to queue a new request even if - it is over the queue limit. This must be used - very carefully!! - -elevator_set_req_fn -elevator_put_req_fn Must be used to allocate and free any elevator - specific storage for a request. - -elevator_activate_req_fn Called when device driver first sees a request. - I/O schedulers can use this callback to - determine when actual execution of a request - starts. -elevator_deactivate_req_fn Called when device driver decides to delay - a request by requeueing it. - -elevator_init_fn* -elevator_exit_fn Allocate and free any elevator specific storage - for a queue. - -4.2 Request flows seen by I/O schedulers -All requests seen by I/O schedulers strictly follow one of the following three -flows. - - set_req_fn -> - - i. add_req_fn -> (merged_fn ->)* -> dispatch_fn -> activate_req_fn -> - (deactivate_req_fn -> activate_req_fn ->)* -> completed_req_fn - ii. add_req_fn -> (merged_fn ->)* -> merge_req_fn - iii. [none] - - -> put_req_fn - -4.3 I/O scheduler implementation -The generic i/o scheduler algorithm attempts to sort/merge/batch requests for -optimal disk scan and request servicing performance (based on generic -principles and device capabilities), optimized for: -i. improved throughput -ii. improved latency -iii. better utilization of h/w & CPU time - -Characteristics: - -i. Binary tree -AS and deadline i/o schedulers use red black binary trees for disk position -sorting and searching, and a fifo linked list for time-based searching. This -gives good scalability and good availability of information. Requests are -almost always dispatched in disk sort order, so a cache is kept of the next -request in sort order to prevent binary tree lookups. - -This arrangement is not a generic block layer characteristic however, so -elevators may implement queues as they please. - -ii. Merge hash -AS and deadline use a hash table indexed by the last sector of a request. This -enables merging code to quickly look up "back merge" candidates, even when -multiple I/O streams are being performed at once on one disk. - -"Front merges", a new request being merged at the front of an existing request, -are far less common than "back merges" due to the nature of most I/O patterns. -Front merges are handled by the binary trees in AS and deadline schedulers. - -iii. Plugging the queue to batch requests in anticipation of opportunities for - merge/sort optimizations - -Plugging is an approach that the current i/o scheduling algorithm resorts to so -that it collects up enough requests in the queue to be able to take -advantage of the sorting/merging logic in the elevator. If the -queue is empty when a request comes in, then it plugs the request queue -(sort of like plugging the bath tub of a vessel to get fluid to build up) -till it fills up with a few more requests, before starting to service -the requests. This provides an opportunity to merge/sort the requests before -passing them down to the device. There are various conditions when the queue is -unplugged (to open up the flow again), either through a scheduled task or -could be on demand. For example wait_on_buffer sets the unplugging going -through sync_buffer() running blk_run_address_space(mapping). Or the caller -can do it explicity through blk_unplug(bdev). So in the read case, -the queue gets explicitly unplugged as part of waiting for completion on that -buffer. - -Aside: - This is kind of controversial territory, as it's not clear if plugging is - always the right thing to do. Devices typically have their own queues, - and allowing a big queue to build up in software, while letting the device be - idle for a while may not always make sense. The trick is to handle the fine - balance between when to plug and when to open up. Also now that we have - multi-page bios being queued in one shot, we may not need to wait to merge - a big request from the broken up pieces coming by. - -4.4 I/O contexts -I/O contexts provide a dynamically allocated per process data area. They may -be used in I/O schedulers, and in the block layer (could be used for IO statis, -priorities for example). See *io_context in block/ll_rw_blk.c, and as-iosched.c -for an example of usage in an i/o scheduler. - - -5. Scalability related changes - -5.1 Granular Locking: io_request_lock replaced by a per-queue lock - -The global io_request_lock has been removed as of 2.5, to avoid -the scalability bottleneck it was causing, and has been replaced by more -granular locking. The request queue structure has a pointer to the -lock to be used for that queue. As a result, locking can now be -per-queue, with a provision for sharing a lock across queues if -necessary (e.g the scsi layer sets the queue lock pointers to the -corresponding adapter lock, which results in a per host locking -granularity). The locking semantics are the same, i.e. locking is -still imposed by the block layer, grabbing the lock before -request_fn execution which it means that lots of older drivers -should still be SMP safe. Drivers are free to drop the queue -lock themselves, if required. Drivers that explicitly used the -io_request_lock for serialization need to be modified accordingly. -Usually it's as easy as adding a global lock: - - static DEFINE_SPINLOCK(my_driver_lock); - -and passing the address to that lock to blk_init_queue(). - -5.2 64 bit sector numbers (sector_t prepares for 64 bit support) - -The sector number used in the bio structure has been changed to sector_t, -which could be defined as 64 bit in preparation for 64 bit sector support. - -6. Other Changes/Implications - -6.1 Partition re-mapping handled by the generic block layer - -In 2.5 some of the gendisk/partition related code has been reorganized. -Now the generic block layer performs partition-remapping early and thus -provides drivers with a sector number relative to whole device, rather than -having to take partition number into account in order to arrive at the true -sector number. The routine blk_partition_remap() is invoked by -generic_make_request even before invoking the queue specific make_request_fn, -so the i/o scheduler also gets to operate on whole disk sector numbers. This -should typically not require changes to block drivers, it just never gets -to invoke its own partition sector offset calculations since all bios -sent are offset from the beginning of the device. - - -7. A Few Tips on Migration of older drivers - -Old-style drivers that just use CURRENT and ignores clustered requests, -may not need much change. The generic layer will automatically handle -clustered requests, multi-page bios, etc for the driver. - -For a low performance driver or hardware that is PIO driven or just doesn't -support scatter-gather changes should be minimal too. - -The following are some points to keep in mind when converting old drivers -to bio. - -Drivers should use elv_next_request to pick up requests and are no longer -supposed to handle looping directly over the request list. -(struct request->queue has been removed) - -Now end_that_request_first takes an additional number_of_sectors argument. -It used to handle always just the first buffer_head in a request, now -it will loop and handle as many sectors (on a bio-segment granularity) -as specified. - -Now bh->b_end_io is replaced by bio->bi_end_io, but most of the time the -right thing to use is bio_endio(bio) instead. - -If the driver is dropping the io_request_lock from its request_fn strategy, -then it just needs to replace that with q->queue_lock instead. - -As described in Sec 1.1, drivers can set max sector size, max segment size -etc per queue now. Drivers that used to define their own merge functions i -to handle things like this can now just use the blk_queue_* functions at -blk_init_queue time. - -Drivers no longer have to map a {partition, sector offset} into the -correct absolute location anymore, this is done by the block layer, so -where a driver received a request ala this before: - - rq->rq_dev = mk_kdev(3, 5); /* /dev/hda5 */ - rq->sector = 0; /* first sector on hda5 */ - - it will now see - - rq->rq_dev = mk_kdev(3, 0); /* /dev/hda */ - rq->sector = 123128; /* offset from start of disk */ - -As mentioned, there is no virtual mapping of a bio. For DMA, this is -not a problem as the driver probably never will need a virtual mapping. -Instead it needs a bus mapping (dma_map_page for a single segment or -use dma_map_sg for scatter gather) to be able to ship it to the driver. For -PIO drivers (or drivers that need to revert to PIO transfer once in a -while (IDE for example)), where the CPU is doing the actual data -transfer a virtual mapping is needed. If the driver supports highmem I/O, -(Sec 1.1, (ii) ) it needs to use kmap_atomic or similar to temporarily map -a bio into the virtual address space. - - -8. Prior/Related/Impacted patches - -8.1. Earlier kiobuf patches (sct/axboe/chait/hch/mkp) -- orig kiobuf & raw i/o patches (now in 2.4 tree) -- direct kiobuf based i/o to devices (no intermediate bh's) -- page i/o using kiobuf -- kiobuf splitting for lvm (mkp) -- elevator support for kiobuf request merging (axboe) -8.2. Zero-copy networking (Dave Miller) -8.3. SGI XFS - pagebuf patches - use of kiobufs -8.4. Multi-page pioent patch for bio (Christoph Hellwig) -8.5. Direct i/o implementation (Andrea Arcangeli) since 2.4.10-pre11 -8.6. Async i/o implementation patch (Ben LaHaise) -8.7. EVMS layering design (IBM EVMS team) -8.8. Larger page cache size patch (Ben LaHaise) and - Large page size (Daniel Phillips) - => larger contiguous physical memory buffers -8.9. VM reservations patch (Ben LaHaise) -8.10. Write clustering patches ? (Marcelo/Quintela/Riel ?) -8.11. Block device in page cache patch (Andrea Archangeli) - now in 2.4.10+ -8.12. Multiple block-size transfers for faster raw i/o (Shailabh Nagar, - Badari) -8.13 Priority based i/o scheduler - prepatches (Arjan van de Ven) -8.14 IDE Taskfile i/o patch (Andre Hedrick) -8.15 Multi-page writeout and readahead patches (Andrew Morton) -8.16 Direct i/o patches for 2.5 using kvec and bio (Badari Pulavarthy) - -9. Other References: - -9.1 The Splice I/O Model - Larry McVoy (and subsequent discussions on lkml, -and Linus' comments - Jan 2001) -9.2 Discussions about kiobuf and bh design on lkml between sct, linus, alan -et al - Feb-March 2001 (many of the initial thoughts that led to bio were -brought up in this discussion thread) -9.3 Discussions on mempool on lkml - Dec 2001. - |