314 lines
11 KiB
Groff
314 lines
11 KiB
Groff
.Dd August 27, 1993
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.Dt ST 4
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.Os FreeBSD
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.Sh NAME
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.Nm st
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.Nd scsi tape driver
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.Sh SYNOPSIS
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.Nm tape st
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.Nm device st1 target 4 lun 0
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.Sh DESCRIPTION
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The
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.Xr st
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driver provides support for a
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.Em scsi
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tape. It allows the tape
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to be run in upto four different modes depending on minor numbers
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and supports several different 'sub modes'.
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The device can have both a
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.Em raw
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interface
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and a
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.Em Block mode
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interface however only the raw interface is usually used (or recommended).
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In general the interfaces are similar to those described by
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.Xr wt 4
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or
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.Xr mt 4 .
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.Pp
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Where the
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.Xr wt 4
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device has a fairly low level interface to the system,
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.Em SCSI
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devices have a much higher level interface and talk to the system via
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a
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.Em SCSI Adapter
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and a
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.Em Scsi Adapter driver
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e.g.
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.Xr AHA1542 .
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A scsi adapter must also be separatly configured into the system
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before a scsi tape can be configured.
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.Pp
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As the scsi adapter is probed during boot, the
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.Em SCSI
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bus is scanned for devices. Any devices found which answer as 'Sequential'
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type devices will be attached to the
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.Nm
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driver.
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In FreeBSD releases prior to 2.1, the first found will be attached as
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.Em cd0
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and the next,
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.Em cd1
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etc.
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Beginning in 2.1 it is possible to specify what cd unit a device should
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come on line as; refer to
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.Xr scsi 4
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for details on kernel configuration.
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.Pp
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.Sh MOUNT SESSIONS
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The
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.Nm
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driver is based around the concept of a
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.Em Mount Session
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, which is defined as the period between the time that a tape is mounted,
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and the time when it is unmounted. Any parameters set during a
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.Em Mount Session
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remain in effect for the remainder of the session or until replaced. The
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Tape can be unmounted, bringing the session to a close in several ways.
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These include:
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.Bl -tag -width ABOUT_THIS_BIG_BUT_REALLY_BIGGER
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.It Pa closing an 'unmount device'
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This is referred to as sub-mode 00 (see below). An example is /dev/rst0.
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.It Pa an MTOFFL ioctl
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Reachable throught the 'offline' command of
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.Xr st 1
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.It Pa Openning another mode.
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Openning a different mode will implicitly unmount the tape, thereby closing
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off the mode that was previously mounted. All parameters will be loaded
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freshly from the new mode. (see below for more on 'modes').
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.El
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.Pp
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Parameters that are required to last across the unmounting of a tape,
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should be set on the control device. This is submode 3 (see below) and is
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reached through a file with a name of the form /dev/st{y}ctl.{x}, where
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{y} is the drive number and {x} is the mode number.
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.Pp
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.Sh MODES AND SUB MODES
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There are four Operation modes. These are controlled by bits 2
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and 3 of the minor number and are designed to allow people to easily
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read and write different formats of tape on devices that allow
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multiple formats. The parameters for each mode can be set individually
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by hand with the
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.Xr st 1
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variant of the
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.Xr mt 1
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command. When a device corresponding to a particular mode is first mounted,
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The operating parameters for that
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.Em Mount Session
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Are copied from that mode. Further changes to the parameters during the
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session will change those in effect for the session but not those set
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in the Operating Mode. To change the parameters for an Operating Mode,
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One must either assign the parameters to the control device, or compile
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them into the 'Rogues Gallery' table within the driver.
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.Pp
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In addition to the four Operation Modes mentionned above,
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bits 0 and 1 of the minor number are interpretted as being 'sub-modes'.
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The following sub-modes are supported
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.Bl -tag -width ABOUT_THIS_BIG
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.It Pa 00
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A close will rewind the device. If the tape has been
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written, then a Filemark will be written before the rewind is requested.
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The device is UNMOUNTED.
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.It Pa 01
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A close will leave the tape MOUNTED.
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If the tape was written to a filemark will be written.
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No other head positioning takes place.
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Any further reads or writes will occur directly after the
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last read, or the written filemark.
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.It Pa 10
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A close will rewind the device. If the tape has been
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written, then a Filemark will be written before the rewind is requested.
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On completion of the rewind an UNLOAD command will be issued.
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The device is UNMOUNTED.
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.It Pa 11
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This is a special mode.
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It is known as the
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.Em CONTROL DEVICE
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for the mode. Parameters set for the mode while in this sub
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mode will be remembered from one mount to the next. This allows the
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system administrator to set different characteristics (e.g. density,
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blocksize, (and eventually compression)) on each mode, and have the
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different modes keep those parameters independent of any parameter
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changes a user may invoke during a single mount session. At the
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completion of the user's mount session, drive parameters will revert
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to those set by the administrator. IO operations cannot be performed
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on this device/submode. General
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.Xr scsi 4
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ioctls
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.Em MUST
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be performed against the control device.
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.El
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.Sh BLOCKING MODES
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Scsi Tapes may run in either 'variable' or 'fixed block' modes.
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Most QIC type devices run in Fixed block mode, where most 'reel to reel' tapes and
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many new cartridge formats, allow variable blocksize. The difference between
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the two is as follows:
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.Bl -tag -width variable-blocksize
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.It Pa Variable Blocksize
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Each write made to the device results in a single logical record
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written to the tape. You can never read or write PART of a record
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from tape, (though you may request a larger block and read a smaller
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record). You cannot read multiple blocks either. Data from a single
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write is therefore read by a single read. The block size used may
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be any value supported by the device, the scsi adapter and the
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system. (often variable between 1 byte and 64k (sometimes more)).
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.Pp
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When reading a variable record/block from the tape, the head is
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logically considered to be immediately after the last item read,
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and before the next item after that. If the next item is a Filemark,
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but you never read it because you have all the data, then the next
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process to read will immediately read the filemark and return EOF.
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(assuming you were in non-rewind mode).
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.It Pa fixed Blocksize
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Data written by the user is passed to the tape as a succession of
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fixed size blocks. It may be contiguouse in ram and read in a single
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DMA pass, however it is considered to be a series of independent
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blocks. You may never write an amount of data that is not an exact
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multiple of the blocksize. You may read and write the same data
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as a different set of records, In other words, blocks that were
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written together may be read separatly, and visa versa.
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.Pp
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If you ask for more blocks than there are left in the file, then
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the drive will encounter the filemark. Because there is some data
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to return to you (unless there were no records before the filemark)
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the driver will return the data to you (less than you requested),
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but hide from you the discovery of the Filemark. The NEXT read
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will be returned immediately with an EOF. If you never Make the next
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read, but close the device, the next process to read will immediately
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read the filemark and return EOF. (assuming you were in non-rewind
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mode).
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.El
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.Sh FILEMARK HANDLING
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The handling of filemarks on write is pretty much automatic. If you
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have written to the tape, and not done a read since, then a filemark will
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be written to the tape when you close the device. If a rewind is requested
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after a write, then the driver assumes that you have written the last file
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on the tape and ensures that there are two filemarks written to the tape.
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It takes into account any filemarks already written (whether by close
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or by explicit ioctl). The exception to this is that there seems to be
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a standard (which we follow, but don't understand why) that certain
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types of tape do not actually write two filemarks to tape,
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but when read, report a 'phantom' filemark when the last file is read.
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These devices include the QIC family of devices. It might be that this
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set of devices is the same set as that of fixed block devices. This has not
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been detirmined yet, and they are treated as separate behaviors by the
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driver at this time.
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.Pp
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.SH KERNEL CONFIGURATION
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In configuring, if an optional
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.Ar count
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is given in
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the specification, that number of scsi tapes are configured;
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Most storage for them is allocated only when found so a large number
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of configured devices is cheap. (once the first has included the driver).
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.Pp
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Because different tape drives behave differently, there is a mechanism
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within the source to st, to quickly and conveniently recognise and deal
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with brands and models of drive that have special requirements.
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.Pp
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There is a table (called the rogues gallery) in which the indentification
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strings of known errant drives can be stored. Along with each is
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a set of flags that allows the setting of densities and blocksizes for each
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of the 4 modes, along with a set of 'QUIRK' flags that can be
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used to enable or disable sections of code within the driver if a particular
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drive is recognised.
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.Pp
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.Sh IOCTLS
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The following
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.Xr ioctl 2
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calls apply to scsi tapes. Some also apply to other tapes. They are defined
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in the header file
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.Em /sys/mtio.h.
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.Bl -tag -width MTIOCEEOT
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.It Pa MTIOCGET
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Get the mt control structure filled out by the driver, showing
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all the present settings.
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.It Pa MTIOCTOP
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Perform one of the following operations. These operations all have a
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single argument, which is either a boolean, or a signed integer, depending
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on the operation.
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.Bl -tag -width MTSELDNSTY
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.It Pa MTWEOF
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Write N end of file marks at the present head position.
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.It Pa MTFSF
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Skip over N Filemarks. Leave the head on the EOM side of the last skipped
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Filemark.
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.It Pa MTBSF
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Skip BACKWARDS over N Filemarks. Leave the head on the BOM (beginning of media)
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side of the last skipped Filemark.
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.It Pa MTFSR
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Skip forwards over N records.
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.It Pa MTBSR
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Skip backwards over N records.
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.It Pa MTREW
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Rewind the device to the beginning of the media.
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.It Pa MTOFFL
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Rewind the media (and if possible eject). Even if the device cannot
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eject the media it will often no longer respond to normal requests.
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.It Pa MTNOP
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No Op, set status only..
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.It Pa MTCACHE
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Enable controller Buffering.
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.It Pa MTNOCACHE
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Disable controller Buffering.
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.It Pa MTSETBSIZ
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Set the blocksize to use for the device/mode. If the device is capable of
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variable blocksize operation, and the blocksize is set to 0, then the drive
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will be driven in variable mode. This parameter is in effect for the present
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mount session only, unless set on the control device.
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.It Pa MTSETDNSTY
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Set the Density value (see
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.Xr st 1
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) to use when running in the mode openned (minor bits 2,3).
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This parameter is in effect for the present
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mount session only, unless set on the control device.
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.El
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.It Pa MTIOCIEOT
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?Set END of TAPE processing... not yet supported.
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.It Pa MTIOCEEOT
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?Set END of TAPE processing... not yet supported.
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.El
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.Pp
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In addition, The
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.Nm
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driver will allow the use of any of the general
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.Xr scsi 4
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ioctls, as long as the control device is used.
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.Sh FILES
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.Bl -tag -width /dev/[n][e]rst[0-9].[0-3] -compact
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.It Pa /dev/[n][e]rst[0-9].[0-3]
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general form:
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.It Pa /dev/rst0.0
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Mode 0, rewind on close
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.It Pa /dev/nrst0.2
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Mode 2, No rewind on close
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.It Pa /dev/erst0.3
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Mode 3, Eject on close (if capable)
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.It Pa /dev/rst0
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Another name for rst0.0
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.It Pa /dev/nrst0
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Another name for nrst0.0
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.It Pa /dev/st0ctl.0
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Parameters set to this device become the default parameters for [en]rst0.0
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.It Pa /dev/st0ctl.1
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Parameters set to this device become the default parameters for [en]rst0.1
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.It Pa /dev/st0ctl.2
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Parameters set to this device become the default parameters for [en]rst0.2
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.It Pa /dev/st0ctl.3
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Parameters set to this device become the default parameters for [en]rst0.3
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.El
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.Sh DIAGNOSTICS
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None.
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.Sh SEE ALSO
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.Xr mt 1
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.Xr st 1
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.Sh HISTORY
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This
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.Nm
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driver appeared in MACH 2.5 .
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