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diff --git a/Documentation/scsi/st.txt b/Documentation/scsi/st.txt deleted file mode 100644 index ec0acf6acccd..000000000000 --- a/Documentation/scsi/st.txt +++ /dev/null @@ -1,592 +0,0 @@ -This file contains brief information about the SCSI tape driver. -The driver is currently maintained by Kai Mäkisara (email -Kai.Makisara@kolumbus.fi) - -Last modified: Tue Feb 9 21:54:16 2016 by kai.makisara - - -BASICS - -The driver is generic, i.e., it does not contain any code tailored -to any specific tape drive. The tape parameters can be specified with -one of the following three methods: - -1. Each user can specify the tape parameters he/she wants to use -directly with ioctls. This is administratively a very simple and -flexible method and applicable to single-user workstations. However, -in a multiuser environment the next user finds the tape parameters in -state the previous user left them. - -2. The system manager (root) can define default values for some tape -parameters, like block size and density using the MTSETDRVBUFFER ioctl. -These parameters can be programmed to come into effect either when a -new tape is loaded into the drive or if writing begins at the -beginning of the tape. The second method is applicable if the tape -drive performs auto-detection of the tape format well (like some -QIC-drives). The result is that any tape can be read, writing can be -continued using existing format, and the default format is used if -the tape is rewritten from the beginning (or a new tape is written -for the first time). The first method is applicable if the drive -does not perform auto-detection well enough and there is a single -"sensible" mode for the device. An example is a DAT drive that is -used only in variable block mode (I don't know if this is sensible -or not :-). - -The user can override the parameters defined by the system -manager. The changes persist until the defaults again come into -effect. - -3. By default, up to four modes can be defined and selected using the minor -number (bits 5 and 6). The number of modes can be changed by changing -ST_NBR_MODE_BITS in st.h. Mode 0 corresponds to the defaults discussed -above. Additional modes are dormant until they are defined by the -system manager (root). When specification of a new mode is started, -the configuration of mode 0 is used to provide a starting point for -definition of the new mode. - -Using the modes allows the system manager to give the users choices -over some of the buffering parameters not directly accessible to the -users (buffered and asynchronous writes). The modes also allow choices -between formats in multi-tape operations (the explicitly overridden -parameters are reset when a new tape is loaded). - -If more than one mode is used, all modes should contain definitions -for the same set of parameters. - -Many Unices contain internal tables that associate different modes to -supported devices. The Linux SCSI tape driver does not contain such -tables (and will not do that in future). Instead of that, a utility -program can be made that fetches the inquiry data sent by the device, -scans its database, and sets up the modes using the ioctls. Another -alternative is to make a small script that uses mt to set the defaults -tailored to the system. - -The driver supports fixed and variable block size (within buffer -limits). Both the auto-rewind (minor equals device number) and -non-rewind devices (minor is 128 + device number) are implemented. - -In variable block mode, the byte count in write() determines the size -of the physical block on tape. When reading, the drive reads the next -tape block and returns to the user the data if the read() byte count -is at least the block size. Otherwise, error ENOMEM is returned. - -In fixed block mode, the data transfer between the drive and the -driver is in multiples of the block size. The write() byte count must -be a multiple of the block size. This is not required when reading but -may be advisable for portability. - -Support is provided for changing the tape partition and partitioning -of the tape with one or two partitions. By default support for -partitioned tape is disabled for each driver and it can be enabled -with the ioctl MTSETDRVBUFFER. - -By default the driver writes one filemark when the device is closed after -writing and the last operation has been a write. Two filemarks can be -optionally written. In both cases end of data is signified by -returning zero bytes for two consecutive reads. - -Writing filemarks without the immediate bit set in the SCSI command block acts -as a synchronization point, i.e., all remaining data form the drive buffers is -written to tape before the command returns. This makes sure that write errors -are caught at that point, but this takes time. In some applications, several -consecutive files must be written fast. The MTWEOFI operation can be used to -write the filemarks without flushing the drive buffer. Writing filemark at -close() is always flushing the drive buffers. However, if the previous -operation is MTWEOFI, close() does not write a filemark. This can be used if -the program wants to close/open the tape device between files and wants to -skip waiting. - -If rewind, offline, bsf, or seek is done and previous tape operation was -write, a filemark is written before moving tape. - -The compile options are defined in the file linux/drivers/scsi/st_options.h. - -4. If the open option O_NONBLOCK is used, open succeeds even if the -drive is not ready. If O_NONBLOCK is not used, the driver waits for -the drive to become ready. If this does not happen in ST_BLOCK_SECONDS -seconds, open fails with the errno value EIO. With O_NONBLOCK the -device can be opened for writing even if there is a write protected -tape in the drive (commands trying to write something return error if -attempted). - - -MINOR NUMBERS - -The tape driver currently supports up to 2^17 drives if 4 modes for -each drive are used. - -The minor numbers consist of the following bit fields: - -dev_upper non-rew mode dev-lower - 20 - 8 7 6 5 4 0 -The non-rewind bit is always bit 7 (the uppermost bit in the lowermost -byte). The bits defining the mode are below the non-rewind bit. The -remaining bits define the tape device number. This numbering is -backward compatible with the numbering used when the minor number was -only 8 bits wide. - - -SYSFS SUPPORT - -The driver creates the directory /sys/class/scsi_tape and populates it with -directories corresponding to the existing tape devices. There are autorewind -and non-rewind entries for each mode. The names are stxy and nstxy, where x -is the tape number and y a character corresponding to the mode (none, l, m, -a). For example, the directories for the first tape device are (assuming four -modes): st0 nst0 st0l nst0l st0m nst0m st0a nst0a. - -Each directory contains the entries: default_blksize default_compression -default_density defined dev device driver. The file 'defined' contains 1 -if the mode is defined and zero if not defined. The files 'default_*' contain -the defaults set by the user. The value -1 means the default is not set. The -file 'dev' contains the device numbers corresponding to this device. The links -'device' and 'driver' point to the SCSI device and driver entries. - -Each directory also contains the entry 'options' which shows the currently -enabled driver and mode options. The value in the file is a bit mask where the -bit definitions are the same as those used with MTSETDRVBUFFER in setting the -options. - -A link named 'tape' is made from the SCSI device directory to the class -directory corresponding to the mode 0 auto-rewind device (e.g., st0). - - -SYSFS AND STATISTICS FOR TAPE DEVICES - -The st driver maintains statistics for tape drives inside the sysfs filesystem. -The following method can be used to locate the statistics that are -available (assuming that sysfs is mounted at /sys): - -1. Use opendir(3) on the directory /sys/class/scsi_tape -2. Use readdir(3) to read the directory contents -3. Use regcomp(3)/regexec(3) to match directory entries to the extended - regular expression "^st[0-9]+$" -4. Access the statistics from the /sys/class/scsi_tape/<match>/stats - directory (where <match> is a directory entry from /sys/class/scsi_tape - that matched the extended regular expression) - -The reason for using this approach is that all the character devices -pointing to the same tape drive use the same statistics. That means -that st0 would have the same statistics as nst0. - -The directory contains the following statistics files: - -1. in_flight - The number of I/Os currently outstanding to this device. -2. io_ns - The amount of time spent waiting (in nanoseconds) for all I/O - to complete (including read and write). This includes tape movement - commands such as seeking between file or set marks and implicit tape - movement such as when rewind on close tape devices are used. -3. other_cnt - The number of I/Os issued to the tape drive other than read or - write commands. The time taken to complete these commands uses the - following calculation io_ms-read_ms-write_ms. -4. read_byte_cnt - The number of bytes read from the tape drive. -5. read_cnt - The number of read requests issued to the tape drive. -6. read_ns - The amount of time (in nanoseconds) spent waiting for read - requests to complete. -7. write_byte_cnt - The number of bytes written to the tape drive. -8. write_cnt - The number of write requests issued to the tape drive. -9. write_ns - The amount of time (in nanoseconds) spent waiting for write - requests to complete. -10. resid_cnt - The number of times during a read or write we found - the residual amount to be non-zero. This should mean that a program - is issuing a read larger thean the block size on tape. For write - not all data made it to tape. - -Note: The in_flight value is incremented when an I/O starts the I/O -itself is not added to the statistics until it completes. - -The total of read_cnt, write_cnt, and other_cnt may not total to the same -value as iodone_cnt at the device level. The tape statistics only count -I/O issued via the st module. - -When read the statistics may not be temporally consistent while I/O is in -progress. The individual values are read and written to atomically however -when reading them back via sysfs they may be in the process of being -updated when starting an I/O or when it is completed. - -The value shown in in_flight is incremented before any statstics are -updated and decremented when an I/O completes after updating statistics. -The value of in_flight is 0 when there are no I/Os outstanding that are -issued by the st driver. Tape statistics do not take into account any -I/O performed via the sg device. - -BSD AND SYS V SEMANTICS - -The user can choose between these two behaviours of the tape driver by -defining the value of the symbol ST_SYSV. The semantics differ when a -file being read is closed. The BSD semantics leaves the tape where it -currently is whereas the SYS V semantics moves the tape past the next -filemark unless the filemark has just been crossed. - -The default is BSD semantics. - - -BUFFERING - -The driver tries to do transfers directly to/from user space. If this -is not possible, a driver buffer allocated at run-time is used. If -direct i/o is not possible for the whole transfer, the driver buffer -is used (i.e., bounce buffers for individual pages are not -used). Direct i/o can be impossible because of several reasons, e.g.: -- one or more pages are at addresses not reachable by the HBA -- the number of pages in the transfer exceeds the number of - scatter/gather segments permitted by the HBA -- one or more pages can't be locked into memory (should not happen in - any reasonable situation) - -The size of the driver buffers is always at least one tape block. In fixed -block mode, the minimum buffer size is defined (in 1024 byte units) by -ST_FIXED_BUFFER_BLOCKS. With small block size this allows buffering of -several blocks and using one SCSI read or write to transfer all of the -blocks. Buffering of data across write calls in fixed block mode is -allowed if ST_BUFFER_WRITES is non-zero and direct i/o is not used. -Buffer allocation uses chunks of memory having sizes 2^n * (page -size). Because of this the actual buffer size may be larger than the -minimum allowable buffer size. - -NOTE that if direct i/o is used, the small writes are not buffered. This may -cause a surprise when moving from 2.4. There small writes (e.g., tar without --b option) may have had good throughput but this is not true any more with -2.6. Direct i/o can be turned off to solve this problem but a better solution -is to use bigger write() byte counts (e.g., tar -b 64). - -Asynchronous writing. Writing the buffer contents to the tape is -started and the write call returns immediately. The status is checked -at the next tape operation. Asynchronous writes are not done with -direct i/o and not in fixed block mode. - -Buffered writes and asynchronous writes may in some rare cases cause -problems in multivolume operations if there is not enough space on the -tape after the early-warning mark to flush the driver buffer. - -Read ahead for fixed block mode (ST_READ_AHEAD). Filling the buffer is -attempted even if the user does not want to get all of the data at -this read command. Should be disabled for those drives that don't like -a filemark to truncate a read request or that don't like backspacing. - -Scatter/gather buffers (buffers that consist of chunks non-contiguous -in the physical memory) are used if contiguous buffers can't be -allocated. To support all SCSI adapters (including those not -supporting scatter/gather), buffer allocation is using the following -three kinds of chunks: -1. The initial segment that is used for all SCSI adapters including -those not supporting scatter/gather. The size of this buffer will be -(PAGE_SIZE << ST_FIRST_ORDER) bytes if the system can give a chunk of -this size (and it is not larger than the buffer size specified by -ST_BUFFER_BLOCKS). If this size is not available, the driver halves -the size and tries again until the size of one page. The default -settings in st_options.h make the driver to try to allocate all of the -buffer as one chunk. -2. The scatter/gather segments to fill the specified buffer size are -allocated so that as many segments as possible are used but the number -of segments does not exceed ST_FIRST_SG. -3. The remaining segments between ST_MAX_SG (or the module parameter -max_sg_segs) and the number of segments used in phases 1 and 2 -are used to extend the buffer at run-time if this is necessary. The -number of scatter/gather segments allowed for the SCSI adapter is not -exceeded if it is smaller than the maximum number of scatter/gather -segments specified. If the maximum number allowed for the SCSI adapter -is smaller than the number of segments used in phases 1 and 2, -extending the buffer will always fail. - - -EOM BEHAVIOUR WHEN WRITING - -When the end of medium early warning is encountered, the current write -is finished and the number of bytes is returned. The next write -returns -1 and errno is set to ENOSPC. To enable writing a trailer, -the next write is allowed to proceed and, if successful, the number of -bytes is returned. After this, -1 and the number of bytes are -alternately returned until the physical end of medium (or some other -error) is encountered. - - -MODULE PARAMETERS - -The buffer size, write threshold, and the maximum number of allocated buffers -are configurable when the driver is loaded as a module. The keywords are: - -buffer_kbs=xxx the buffer size for fixed block mode is set - to xxx kilobytes -write_threshold_kbs=xxx the write threshold in kilobytes set to xxx -max_sg_segs=xxx the maximum number of scatter/gather - segments -try_direct_io=x try direct transfer between user buffer and - tape drive if this is non-zero - -Note that if the buffer size is changed but the write threshold is not -set, the write threshold is set to the new buffer size - 2 kB. - - -BOOT TIME CONFIGURATION - -If the driver is compiled into the kernel, the same parameters can be -also set using, e.g., the LILO command line. The preferred syntax is -to use the same keyword used when loading as module but prepended -with 'st.'. For instance, to set the maximum number of scatter/gather -segments, the parameter 'st.max_sg_segs=xx' should be used (xx is the -number of scatter/gather segments). - -For compatibility, the old syntax from early 2.5 and 2.4 kernel -versions is supported. The same keywords can be used as when loading -the driver as module. If several parameters are set, the keyword-value -pairs are separated with a comma (no spaces allowed). A colon can be -used instead of the equal mark. The definition is prepended by the -string st=. Here is an example: - - st=buffer_kbs:64,write_threshold_kbs:60 - -The following syntax used by the old kernel versions is also supported: - - st=aa[,bb[,dd]] - -where - aa is the buffer size for fixed block mode in 1024 byte units - bb is the write threshold in 1024 byte units - dd is the maximum number of scatter/gather segments - - -IOCTLS - -The tape is positioned and the drive parameters are set with ioctls -defined in mtio.h The tape control program 'mt' uses these ioctls. Try -to find an mt that supports all of the Linux SCSI tape ioctls and -opens the device for writing if the tape contents will be modified -(look for a package mt-st* from the Linux ftp sites; the GNU mt does -not open for writing for, e.g., erase). - -The supported ioctls are: - -The following use the structure mtop: - -MTFSF Space forward over count filemarks. Tape positioned after filemark. -MTFSFM As above but tape positioned before filemark. -MTBSF Space backward over count filemarks. Tape positioned before - filemark. -MTBSFM As above but ape positioned after filemark. -MTFSR Space forward over count records. -MTBSR Space backward over count records. -MTFSS Space forward over count setmarks. -MTBSS Space backward over count setmarks. -MTWEOF Write count filemarks. -MTWEOFI Write count filemarks with immediate bit set (i.e., does not - wait until data is on tape) -MTWSM Write count setmarks. -MTREW Rewind tape. -MTOFFL Set device off line (often rewind plus eject). -MTNOP Do nothing except flush the buffers. -MTRETEN Re-tension tape. -MTEOM Space to end of recorded data. -MTERASE Erase tape. If the argument is zero, the short erase command - is used. The long erase command is used with all other values - of the argument. -MTSEEK Seek to tape block count. Uses Tandberg-compatible seek (QFA) - for SCSI-1 drives and SCSI-2 seek for SCSI-2 drives. The file and - block numbers in the status are not valid after a seek. -MTSETBLK Set the drive block size. Setting to zero sets the drive into - variable block mode (if applicable). -MTSETDENSITY Sets the drive density code to arg. See drive - documentation for available codes. -MTLOCK and MTUNLOCK Explicitly lock/unlock the tape drive door. -MTLOAD and MTUNLOAD Explicitly load and unload the tape. If the - command argument x is between MT_ST_HPLOADER_OFFSET + 1 and - MT_ST_HPLOADER_OFFSET + 6, the number x is used sent to the - drive with the command and it selects the tape slot to use of - HP C1553A changer. -MTCOMPRESSION Sets compressing or uncompressing drive mode using the - SCSI mode page 15. Note that some drives other methods for - control of compression. Some drives (like the Exabytes) use - density codes for compression control. Some drives use another - mode page but this page has not been implemented in the - driver. Some drives without compression capability will accept - any compression mode without error. -MTSETPART Moves the tape to the partition given by the argument at the - next tape operation. The block at which the tape is positioned - is the block where the tape was previously positioned in the - new active partition unless the next tape operation is - MTSEEK. In this case the tape is moved directly to the block - specified by MTSEEK. MTSETPART is inactive unless - MT_ST_CAN_PARTITIONS set. -MTMKPART Formats the tape with one partition (argument zero) or two - partitions (argument non-zero). If the argument is positive, - it specifies the size of partition 1 in megabytes. For DDS - drives and several early drives this is the physically first - partition of the tape. If the argument is negative, its absolute - value specifies the size of partition 0 in megabytes. This is - the physically first partition of many later drives, like the - LTO drives from LTO-5 upwards. The drive has to support partitions - with size specified by the initiator. Inactive unless - MT_ST_CAN_PARTITIONS set. -MTSETDRVBUFFER - Is used for several purposes. The command is obtained from count - with mask MT_SET_OPTIONS, the low order bits are used as argument. - This command is only allowed for the superuser (root). The - subcommands are: - 0 - The drive buffer option is set to the argument. Zero means - no buffering. - MT_ST_BOOLEANS - Sets the buffering options. The bits are the new states - (enabled/disabled) the following options (in the - parenthesis is specified whether the option is global or - can be specified differently for each mode): - MT_ST_BUFFER_WRITES write buffering (mode) - MT_ST_ASYNC_WRITES asynchronous writes (mode) - MT_ST_READ_AHEAD read ahead (mode) - MT_ST_TWO_FM writing of two filemarks (global) - MT_ST_FAST_EOM using the SCSI spacing to EOD (global) - MT_ST_AUTO_LOCK automatic locking of the drive door (global) - MT_ST_DEF_WRITES the defaults are meant only for writes (mode) - MT_ST_CAN_BSR backspacing over more than one records can - be used for repositioning the tape (global) - MT_ST_NO_BLKLIMS the driver does not ask the block limits - from the drive (block size can be changed only to - variable) (global) - MT_ST_CAN_PARTITIONS enables support for partitioned - tapes (global) - MT_ST_SCSI2LOGICAL the logical block number is used in - the MTSEEK and MTIOCPOS for SCSI-2 drives instead of - the device dependent address. It is recommended to set - this flag unless there are tapes using the device - dependent (from the old times) (global) - MT_ST_SYSV sets the SYSV semantics (mode) - MT_ST_NOWAIT enables immediate mode (i.e., don't wait for - the command to finish) for some commands (e.g., rewind) - MT_ST_NOWAIT_EOF enables immediate filemark mode (i.e. when - writing a filemark, don't wait for it to complete). Please - see the BASICS note about MTWEOFI with respect to the - possible dangers of writing immediate filemarks. - MT_ST_SILI enables setting the SILI bit in SCSI commands when - reading in variable block mode to enhance performance when - reading blocks shorter than the byte count; set this only - if you are sure that the drive supports SILI and the HBA - correctly returns transfer residuals - MT_ST_DEBUGGING debugging (global; debugging must be - compiled into the driver) - MT_ST_SETBOOLEANS - MT_ST_CLEARBOOLEANS - Sets or clears the option bits. - MT_ST_WRITE_THRESHOLD - Sets the write threshold for this device to kilobytes - specified by the lowest bits. - MT_ST_DEF_BLKSIZE - Defines the default block size set automatically. Value - 0xffffff means that the default is not used any more. - MT_ST_DEF_DENSITY - MT_ST_DEF_DRVBUFFER - Used to set or clear the density (8 bits), and drive buffer - state (3 bits). If the value is MT_ST_CLEAR_DEFAULT - (0xfffff) the default will not be used any more. Otherwise - the lowermost bits of the value contain the new value of - the parameter. - MT_ST_DEF_COMPRESSION - The compression default will not be used if the value of - the lowermost byte is 0xff. Otherwise the lowermost bit - contains the new default. If the bits 8-15 are set to a - non-zero number, and this number is not 0xff, the number is - used as the compression algorithm. The value - MT_ST_CLEAR_DEFAULT can be used to clear the compression - default. - MT_ST_SET_TIMEOUT - Set the normal timeout in seconds for this device. The - default is 900 seconds (15 minutes). The timeout should be - long enough for the retries done by the device while - reading/writing. - MT_ST_SET_LONG_TIMEOUT - Set the long timeout that is used for operations that are - known to take a long time. The default is 14000 seconds - (3.9 hours). For erase this value is further multiplied by - eight. - MT_ST_SET_CLN - Set the cleaning request interpretation parameters using - the lowest 24 bits of the argument. The driver can set the - generic status bit GMT_CLN if a cleaning request bit pattern - is found from the extended sense data. Many drives set one or - more bits in the extended sense data when the drive needs - cleaning. The bits are device-dependent. The driver is - given the number of the sense data byte (the lowest eight - bits of the argument; must be >= 18 (values 1 - 17 - reserved) and <= the maximum requested sense data sixe), - a mask to select the relevant bits (the bits 9-16), and the - bit pattern (bits 17-23). If the bit pattern is zero, one - or more bits under the mask indicate cleaning request. If - the pattern is non-zero, the pattern must match the masked - sense data byte. - - (The cleaning bit is set if the additional sense code and - qualifier 00h 17h are seen regardless of the setting of - MT_ST_SET_CLN.) - -The following ioctl uses the structure mtpos: -MTIOCPOS Reads the current position from the drive. Uses - Tandberg-compatible QFA for SCSI-1 drives and the SCSI-2 - command for the SCSI-2 drives. - -The following ioctl uses the structure mtget to return the status: -MTIOCGET Returns some status information. - The file number and block number within file are returned. The - block is -1 when it can't be determined (e.g., after MTBSF). - The drive type is either MTISSCSI1 or MTISSCSI2. - The number of recovered errors since the previous status call - is stored in the lower word of the field mt_erreg. - The current block size and the density code are stored in the field - mt_dsreg (shifts for the subfields are MT_ST_BLKSIZE_SHIFT and - MT_ST_DENSITY_SHIFT). - The GMT_xxx status bits reflect the drive status. GMT_DR_OPEN - is set if there is no tape in the drive. GMT_EOD means either - end of recorded data or end of tape. GMT_EOT means end of tape. - - -MISCELLANEOUS COMPILE OPTIONS - -The recovered write errors are considered fatal if ST_RECOVERED_WRITE_FATAL -is defined. - -The maximum number of tape devices is determined by the define -ST_MAX_TAPES. If more tapes are detected at driver initialization, the -maximum is adjusted accordingly. - -Immediate return from tape positioning SCSI commands can be enabled by -defining ST_NOWAIT. If this is defined, the user should take care that -the next tape operation is not started before the previous one has -finished. The drives and SCSI adapters should handle this condition -gracefully, but some drive/adapter combinations are known to hang the -SCSI bus in this case. - -The MTEOM command is by default implemented as spacing over 32767 -filemarks. With this method the file number in the status is -correct. The user can request using direct spacing to EOD by setting -ST_FAST_EOM 1 (or using the MT_ST_OPTIONS ioctl). In this case the file -number will be invalid. - -When using read ahead or buffered writes the position within the file -may not be correct after the file is closed (correct position may -require backspacing over more than one record). The correct position -within file can be obtained if ST_IN_FILE_POS is defined at compile -time or the MT_ST_CAN_BSR bit is set for the drive with an ioctl. -(The driver always backs over a filemark crossed by read ahead if the -user does not request data that far.) - - -DEBUGGING HINTS - -Debugging code is now compiled in by default but debugging is turned off -with the kernel module parameter debug_flag defaulting to 0. Debugging -can still be switched on and off with an ioctl. To enable debug at -module load time add debug_flag=1 to the module load options, the -debugging output is not voluminous. Debugging can also be enabled -and disabled by writing a '0' (disable) or '1' (enable) to the sysfs -file /sys/bus/scsi/drivers/st/debug_flag. - -If the tape seems to hang, I would be very interested to hear where -the driver is waiting. With the command 'ps -l' you can see the state -of the process using the tape. If the state is D, the process is -waiting for something. The field WCHAN tells where the driver is -waiting. If you have the current System.map in the correct place (in -/boot for the procps I use) or have updated /etc/psdatabase (for kmem -ps), ps writes the function name in the WCHAN field. If not, you have -to look up the function from System.map. - -Note also that the timeouts are very long compared to most other -drivers. This means that the Linux driver may appear hung although the -real reason is that the tape firmware has got confused. |