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-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: Sun Aug 29 18:25:47 2010 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 128 drives by default. This number
-can be increased by editing st.h and recompiling the driver if
-necessary. The upper limit is 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).
-
-
-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 (the argument gives in megabytes the size of
- partition 1 that is physically the first partition of the
- tape). 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
-
-To enable debugging messages, edit st.c and #define DEBUG 1. As seen
-above, debugging can be switched off with an ioctl if debugging is
-compiled into the driver. The debugging output is not voluminous.
-
-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.