scsi_tape.7 (2010 09)

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scsi_tape(7) scsi_tape(7)

NAME

scsi_tape - SCSI sequential access device driver

DESCRIPTION

SCSI sequential-access (tape) devices store a sequence of data blocks. Data can be read and written using

either ﬁxed or variable sized block mode. If supported by the device, variable sized block mode is nor-

mally used (even when all blocks are the same size). Fixed sized block mode is generally only used for

tape devices which do not support variable sized blocks. Fixed sized block mode can be used on some

tape devices which support variable sized blocks to increase I/O performance.

Generally SCSI tape devices are controlled through the

mt (see mt(7)) generic tape device interface. This

section describes features that are speciﬁc to SCSI tape devices.

The

SIOC_CAPACITY

ioctl (see scsi (7)) can be used to determine remaining tape capacity for some tape

devices. The

blksz ﬁeld indicates the "natural" block size of the device. This value may or may not be

the current block size of the device. The number of blocks, indicated by the

lba ﬁeld, is an estimate of

how much data can be written on the remaining media. A zero size is returned for devices that do not

provide remaining-capacity information. The quantity of data that can actually be written may be higher

or lower than indicated, depending on such factors as block size, media defects, data compression, and

ability to maintain streaming.

To improve performance, most SCSI tape devices have caches. Read-cache use, called "read ahead",

causes the tape drive to read data in anticipation of read requests. Read ahead is only apparent to users

in the increased performance that it produces. Write-cache use is called "immediate reporting". Immedi-

ate reporting increases I/O performance by reporting a completed write status before the data being writ-

ten is actually committed to media. This allows the application program to supply additional data so that

continuous media motion, called "streaming", can be achieved. The

SIOC_GET_IR ioctl can be used to

determine if immediate-reporting functionality is currently being used by the device. The value "1" indi-

cates immediate reporting is enabled. By default, the device driver attempts to enable immediate report-

ing. The

SIOC_SET_IR ioctl can be used to explicitly enable or disable immediate reporting. A zero

value disables immediate reporting. The value "1" enables immediate reporting. The MTIOCTOP ioctl

MTNOP command can be used to cause any cached data to be written (committed) to media. Note that the

device immediate reporting mode set by the SIOC_SET_IR ioctl survives between close() and

open() calls, but not through system reboot.

The

SIOC_GET_BLOCK_SIZE

ioctl indicates the device’s current block size. A block size of zero indi-

cates the device is in variable-sized-block mode. A non-zero block size indicates the device is in ﬁxed-

sized-block mode.

The

SIOC_SET_BLOCK_SIZE

ioctl changes the current block size to the speciﬁed number of bytes.

Setting the block size to zero speciﬁes that variable-sized-block mode should be used. Any non-zero block

size speciﬁes that ﬁxed-sized-block mode should be used. By default, the device driver attempts to set the

block size to zero during open. If variable-sized-block mode is not supported by the device, the driver

selects an appropriate block size for ﬁxed-sized-block mode use. Note that the device block size set by the

SIOC_SET_BLOCK_SIZE

ioctl survives between close() and open() calls, but not through system

reboot.

The

SIOC_GET_BLOCK_LIMITS ioctl indicates the device’s maximum and minimum ﬁxed block-size

limits. The device’s minimum ﬁxed block size is indicated by the min_blk_size ﬁeld. The

max_blk_size ﬁeld contains the smaller of the maximum block size supported by the device and the

maximum block size supported by the system (MAXPHYS). This is the largest valid block size for the

speciﬁc combination of device, driver, and host system being used.

The

SIOC_GET_POSITION ioctl can be used to determine the current media position for some devices.

For devices that support this capability, the resultant value can be used to reposition the media to the

same position in the future.

The

SIOC_SET_POSITION ioctl can be used to cause media repositioning on some devices. For devices

that support this capability, media repositioning via this mechanism can generally be completed more

quickly than might be similarly accomplished using record, ﬁlemark, or setmark spacing. The argument

value speciﬁed should be the result of a previous SIOC_GET_POSITION for that media volume.

The following is included from

<sys/scsi.h>:

/* ioctl support for SCSI tape commands */

#define SIOC_GET_IR _IOR(’S’, 14, int)

#define SIOC_SET_IR _IOW(’S’, 15, int)

HP-UX 11i Version 3: September 2010 − 1 − Hewlett-Packard Company 1

Summary of content (2 pages)

PAGE 1
scsi_tape(7) scsi_tape(7) NAME scsi_tape - SCSI sequential access device driver DESCRIPTION SCSI sequential-access (tape) devices store a sequence of data blocks. Data can be read and written using either fixed or variable sized block mode. If supported by the device, variable sized block mode is normally used (even when all blocks are the same size). Fixed sized block mode is generally only used for tape devices which do not support variable sized blocks.
PAGE 2
scsi_tape(7) #define #define #define #define #define scsi_tape(7) SIOC_GET_BLOCK_SIZE SIOC_SET_BLOCK_SIZE SIOC_GET_BLOCK_LIMITS SIOC_GET_POSITION SIOC_SET_POSITION _IOR(’S’, _IOW(’S’, _IOW(’S’, _IOR(’S’, _IOW(’S’, 30, 31, 32, 33, 34, int) int) struct scsi_block_limits) int) int) /* structure for SIOC_GET_BLOCK_LIMITS ioctl */ struct scsi_block_limits { unsigned min_blk_size; unsigned max_blk_size; }; WARNINGS SCSI bus and device resets cause some devices to reposition media to beginning-of-tape (BOT)