scsi_tape(7) scsi_tape(7)
Series 700 Only
NAME [Toc] [Back]
scsi_tape - SCSI sequential access (tape) device driver
DESCRIPTION [Toc] [Back]
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. 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(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. Readcache
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". Immediate reporting increases I/O
performance by reporting a completed write status before the data
being written 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'' indicates
immediate reporting is enabled. By default, the device driver
attempts to enable immediate reporting. 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 indicates the device is in variablesized-block
mode. A non-zero block size indicates the device is in
Hewlett-Packard Company - 1 - HP-UX 11i Version 2: August 2003
scsi_tape(7) scsi_tape(7)
Series 700 Only
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-sizedblock
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)
#define SIOC_GET_BLOCK_SIZE _IOR('S', 30, int)
#define SIOC_SET_BLOCK_SIZE _IOW('S', 31, int)
#define SIOC_GET_BLOCK_LIMITS _IOW('S', 32, struct scsi_block_limits)
#define SIOC_GET_POSITION _IOR('S', 33, int)
#define SIOC_SET_POSITION _IOW('S', 34, int)
/* structure for SIOC_GET_BLOCK_LIMITS ioctl */
struct scsi_block_limits {
unsigned min_blk_size;
unsigned max_blk_size;
};
Hewlett-Packard Company - 2 - HP-UX 11i Version 2: August 2003
scsi_tape(7) scsi_tape(7)
Series 700 Only
WARNINGS [Toc] [Back]
SCSI bus and device resets cause some devices to reposition media to
beginning-of-tape (BOT). This unintentional media repositioning can
cause loss of data. The scsi_tape driver causes the first subsequent
open() attempt to fail as an indication of potential data loss.
The scsi_tape driver does not write filemarks at close if the media
has been programatically repositioned. Applications that reposition
the media prior to closing the device should write any required
tapemarks.
SEE ALSO [Toc] [Back]
scsi(7), mt(7), mknod(1M).
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