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This file contains brief information about the SCSI tape driver.

The driver is currently maintained by Kai M{kisara (email

Kai.Makisara@metla.fi)

Last modified: Sun Jun 30 15:47:14 1996 by root@kai.makisara.fi

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. Up to four modes can be defined and selected using the minor number

(bits 5 and 6). 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.

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.

The compile options are defined in the file linux/drivers/scsi/st_options.h.

BUFFERING

The driver uses tape buffers allocated either at system initialization

or at run-time when needed. One buffer is used for each open tape

device. The size of the buffers is selectable at compile and/or boot

time. The buffers are used to store the data being transferred to/from

the SCSI adapter. The following buffering options are selectable at

compile time and/or at run time (via ioctl):

Buffering of data across write calls in fixed block mode (define

ST_BUFFER_WRITES).

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.

Buffered writes and asynchronous writes may in some rare cases cause

problems in multivolume operations if there is not enough space 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.

The buffer size is defined (in 1024 byte units) by ST_BUFFER_BLOCKS or

at boot time. If this size is not enough, the driver tries to allocate

a large enough temporary buffer that is released when the device is

closed. The maximum buffer size is defined by the kernel memory

allocation (currently 256 kB for Alphas and 128 kB for other

architectures).

Allocation of the buffers is done at run-time when they are

needed. Allocation of the specified number of buffers can be done at

initialization if ST_RUNTIME_BUFFERS is defined non-zero. The

advantage of run-time allocation is that memory is not wasted for

buffers not being used. The disadvantage is that there may not be

memory available at the time when a buffer is needed for the first

time (once a buffer is allocated, it is not released).

The maximum number of buffers allocated at initialization is defined by

ST_MAX_BUFFERS. One buffer is allocated for each drive detected when

the driver is initialized up to the maximum. The minimum number of

allocated buffers is ST_EXTRA_DEVS (in hosts.h). This ensures some

functionality also for the drives found after tape driver

initialization (a SCSI adapter driver is loaded as a module). The

default for ST_EXTRA_DEVS is two. The driver tries to allocate new

buffers at run-time if necessary.

The threshold for triggering asynchronous write in fixed block mode

is defined by ST_WRITE_THRESHOLD. This may be optimized for each

use pattern. The default triggers asynchronous write after three

default sized writes (10 kB) from tar.

BOOT TIME CONFIGURATION

The buffer size, write threshold, and the maximum number of allocated buffers

are configurable at boot time using, e.g., the LILO command line. The option

syntax is the following:

           st=aa[,bb[,cc]]

where

  aa is the buffer size in 1024 byte units

  bb is the write threshold in 1024 byte units

  cc is the maximum number of tape buffers to allocate (the number of

        buffers is bounded also by the number of drives detected)

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.

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.

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:

           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_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

        MT_ST_DEF_COMPRESSION

           Used to set or clear the density (8 bits), drive buffer

           state (3 bits), and compression (single bit). If the value is

           MT_ST_CLEAR_DEFAULT (0xfffff), the default will not be used

           any more. Otherwise the lower-most bits of the value contain

           the new value of the parameter.

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.

The following ioctls use the structure mtlocation that contains both

the block number and the partition number. These ioctls are available

only for SCSI-2 tape drives and the block number is the

device-independent logical block number defined by the standard.

MTGETLOC Returns the current block and partition number.

MTSETLOC Sets the tape to the block and partition specified by the

        arguments.

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.)