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[pandora-kernel.git] / Documentation / scsi / st.txt

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 Feb 24 21:59:07 2008 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.

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.
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_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.