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Diffstat (limited to 'Documentation/device-mapper/log-writes.txt')
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diff --git a/Documentation/device-mapper/log-writes.txt b/Documentation/device-mapper/log-writes.txt deleted file mode 100644 index b638d124be6a..000000000000 --- a/Documentation/device-mapper/log-writes.txt +++ /dev/null @@ -1,140 +0,0 @@ -dm-log-writes -============= - -This target takes 2 devices, one to pass all IO to normally, and one to log all -of the write operations to. This is intended for file system developers wishing -to verify the integrity of metadata or data as the file system is written to. -There is a log_write_entry written for every WRITE request and the target is -able to take arbitrary data from userspace to insert into the log. The data -that is in the WRITE requests is copied into the log to make the replay happen -exactly as it happened originally. - -Log Ordering -============ - -We log things in order of completion once we are sure the write is no longer in -cache. This means that normal WRITE requests are not actually logged until the -next REQ_PREFLUSH request. This is to make it easier for userspace to replay -the log in a way that correlates to what is on disk and not what is in cache, -to make it easier to detect improper waiting/flushing. - -This works by attaching all WRITE requests to a list once the write completes. -Once we see a REQ_PREFLUSH request we splice this list onto the request and once -the FLUSH request completes we log all of the WRITEs and then the FLUSH. Only -completed WRITEs, at the time the REQ_PREFLUSH is issued, are added in order to -simulate the worst case scenario with regard to power failures. Consider the -following example (W means write, C means complete): - -W1,W2,W3,C3,C2,Wflush,C1,Cflush - -The log would show the following - -W3,W2,flush,W1.... - -Again this is to simulate what is actually on disk, this allows us to detect -cases where a power failure at a particular point in time would create an -inconsistent file system. - -Any REQ_FUA requests bypass this flushing mechanism and are logged as soon as -they complete as those requests will obviously bypass the device cache. - -Any REQ_OP_DISCARD requests are treated like WRITE requests. Otherwise we would -have all the DISCARD requests, and then the WRITE requests and then the FLUSH -request. Consider the following example: - -WRITE block 1, DISCARD block 1, FLUSH - -If we logged DISCARD when it completed, the replay would look like this - -DISCARD 1, WRITE 1, FLUSH - -which isn't quite what happened and wouldn't be caught during the log replay. - -Target interface -================ - -i) Constructor - - log-writes <dev_path> <log_dev_path> - - dev_path : Device that all of the IO will go to normally. - log_dev_path : Device where the log entries are written to. - -ii) Status - - <#logged entries> <highest allocated sector> - - #logged entries : Number of logged entries - highest allocated sector : Highest allocated sector - -iii) Messages - - mark <description> - - You can use a dmsetup message to set an arbitrary mark in a log. - For example say you want to fsck a file system after every - write, but first you need to replay up to the mkfs to make sure - we're fsck'ing something reasonable, you would do something like - this: - - mkfs.btrfs -f /dev/mapper/log - dmsetup message log 0 mark mkfs - <run test> - - This would allow you to replay the log up to the mkfs mark and - then replay from that point on doing the fsck check in the - interval that you want. - - Every log has a mark at the end labeled "dm-log-writes-end". - -Userspace component -=================== - -There is a userspace tool that will replay the log for you in various ways. -It can be found here: https://github.com/josefbacik/log-writes - -Example usage -============= - -Say you want to test fsync on your file system. You would do something like -this: - -TABLE="0 $(blockdev --getsz /dev/sdb) log-writes /dev/sdb /dev/sdc" -dmsetup create log --table "$TABLE" -mkfs.btrfs -f /dev/mapper/log -dmsetup message log 0 mark mkfs - -mount /dev/mapper/log /mnt/btrfs-test -<some test that does fsync at the end> -dmsetup message log 0 mark fsync -md5sum /mnt/btrfs-test/foo -umount /mnt/btrfs-test - -dmsetup remove log -replay-log --log /dev/sdc --replay /dev/sdb --end-mark fsync -mount /dev/sdb /mnt/btrfs-test -md5sum /mnt/btrfs-test/foo -<verify md5sum's are correct> - -Another option is to do a complicated file system operation and verify the file -system is consistent during the entire operation. You could do this with: - -TABLE="0 $(blockdev --getsz /dev/sdb) log-writes /dev/sdb /dev/sdc" -dmsetup create log --table "$TABLE" -mkfs.btrfs -f /dev/mapper/log -dmsetup message log 0 mark mkfs - -mount /dev/mapper/log /mnt/btrfs-test -<fsstress to dirty the fs> -btrfs filesystem balance /mnt/btrfs-test -umount /mnt/btrfs-test -dmsetup remove log - -replay-log --log /dev/sdc --replay /dev/sdb --end-mark mkfs -btrfsck /dev/sdb -replay-log --log /dev/sdc --replay /dev/sdb --start-mark mkfs \ - --fsck "btrfsck /dev/sdb" --check fua - -And that will replay the log until it sees a FUA request, run the fsck command -and if the fsck passes it will replay to the next FUA, until it is completed or -the fsck command exists abnormally. |