scsi_disk(7) scsi_disk(7)
NAME [Toc] [Back]
scsi_disk - SCSI direct access device driver (sdisk)
DESCRIPTION [Toc] [Back]
This section describes the interface for access of SCSI disk, CD-ROM,
and optical disk devices through the character special device driver.
SCSI direct access devices store a sequence of data blocks. Each
direct access device has a specific device size consisting of a number
of data blocks and a logical block size. All data blocks have the
same logical block size. Since I/O operations must have a size that
is an integral number of blocks, one logical block size is the
smallest possible I/O quantity. The device block size can be
determined through use of the DIOC_DESCRIBE, DIOC_CAPACITY, and
SIOC_CAPACITY ioctls (see disk(7) and scsi(7); SIOC_CAPACITY is not
supported on disc3). A direct access device that is not ready for
use, whether due to no media installed or another reason, is
interpreted to mean the device has zero size. An open() call to such
a device succeeds, but subsequent read() and write() calls fail.
The ioctl(2) manpage explains how the operations and arguments are
used. Note, the arg used is commonly the address of the parameter
cited in the particular ioctl #define statement. See the EXAMPLES
section for sample code.
To improve performance, many SCSI disk devices have caches, which can
be used for both read and write operations. Read cache use, called
"read ahead", causes the disk 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. If the subsequent physical write
operation does not complete successfully, data may be lost. Physical
write failures due to media defects are largely eliminated by use of
automatic sparing in disk drives. Power failure between immediate
reporting and media commit can result in cached data being lost.
However, the period of time between these events is typically
relatively small, making such losses unlikely. 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. The value zero indicates immediate reporting is
disabled. The SIOC_SET_IR ioctl can be used to enable or disable
immediate reporting. A zero value disables immediate reporting. The
value 1 enables immediate reporting.
The SIOC_SYNC_CACHE ioctl can be used to force data cached in the
device to media.
Most SCSI removable media disk devices support "prevent" and "allow"
media-removal commands. To avoid data corruption and data
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accessibility problems, media removal is prevented for the entire
duration a removable media disk device is open. Because media removal
is not supported, the SIOC_MEDIUM_CHANGED ioctl is not supported.
The header file <sys/scsi.h> has useful information for direct access
device control, including the following:
/* ioctl support for SCSI disk devices */
#define SIOC_GET_IR _IOR('S', 14, int)
#define SIOC_SET_IR _IOW('S', 15, int)
#define SIOC_SYNC_CACHE -IOW('S', 70, int)
The SIOC_FORMAT ioctl reformats the entire media surface. Exclusive
access to the device, obtained through use of the DIOC_EXCLUSIVE ioctl
(see disk(7)), is required prior to reformatting to ensure that other
applications are not affected. The fmt_optn field can be used to
select the desired media geometry. Only one media geometry is
supported on most devices. The value zero should be used for these
devices. The value zero can also be used to select the default
geometry on devices that support multiple media geometries. The
interleave field can be used to specify sector interleaving. The
value zero specifies that an appropriate default interleave should be
used.
EXAMPLES [Toc] [Back]
The following sample code shows how to use ioctls that affect
scsi_disk.
#include <stdio.h>
#include <fcntl.h>
#include <sys/errno.h>
#include <sys/diskio.h>
#include <sys/scsi.h>
Describe (dfd)
int dfd;
{
int ret;
disk_describe_type descr_type;
if ((ret = ioctl (dfd, DIOC_DESCRIBE, &descr_type)) != 0) {
exit(1);
}
printf ("\nSuccessful ioctl DIOC_DESCRIBE \n");
printf (" model number: %s\n", descr_type.model_num);
printf (" interface: %d <20=scsi>\n", descr_type.intf_type);
}
Exclusive (dfd)
int dfd;
{
int ret, flag=1;
if ((ret = ioctl (dfd, DIOC_EXCLUSIVE, &flag)) != 0) {
exit(1);
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}
}
Enable_WOE (dfd)
int dfd;
{
int ret, flag=1;
if ((ret = ioctl (dfd, SIOC_WRITE_WOE, &flag)) != 0) {
exit(1);
}
printf ("\nSuccessful ioctl SIOC_WRITE_WOE \n");
}
main (argc, argv)
int argc;
char ** argv;
{ int ret, fd; if (argc != 2) {
printf ("Usage: %s <disk3_device> \n", argv[0]);
exit(1);
}
if ((fd = open (argv[1], O_RDWR)) < 0) {
exit (1);
}
Describe (fd);
Exclusive (fd);
Enable_WOE (fd);
}
WARNINGS [Toc] [Back]
Historically, disk devices have had small (typically 512 byte) block
sizes; however, many newer disk devices (such as optical disks and
disk arrays) have relatively large block sizes. Applications using
direct raw disk access should use the DIOC_DESCRIBE, DIOC_CAPACITY, or
SIOC_CAPACITY ioctl to determine the appropriate minimum I/O size.
Media removal and insertion while a disk device is open is unsupported
and unpredictable. Do not attempt to circumvent prevention of media
removal. Device capacity changes resulting from such intervention may
not be recognized.
Often larger I/O operation sizes are expected to be more efficient.
However, SCSI disk I/O operations that are large relative to the
device's cache can result in insufficient cache space for the device
to maintain full-media-speed data transfer rates. This can result in
decreased I/O performance relative to smaller I/O sizes.
DEPENDENCIES [Toc] [Back]
Optical Disk Devices
The SIOC_VERIFY_WRITES ioctl controls the write mode. Normally
written data is assumed to be correctly stored on the media. Verifywrites
mode causes verification of written data to ensure that data
has been correctly written. Verification can substantially reduce
write performance and is not generally needed. The SIOC_VERIFY_WRITES
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scsi_disk(7) scsi_disk(7)
ioctl can be used to enable or disable write verification. A zero
value disables write verification. The value 1 enables write
verification. Although write verification is primarily intended for
optical media, some systems may support write verification on normal
disk devices.
The SIOC_VERIFY ioctl verifies that a media area contains valid data
(that is, data that has been correctly written). Verified media will
not cause I/O errors when reading is attempted. The media area to be
verified is specified via the start_lba and block_cnt fields.
Although verification is intended primarily for optical media, some
systems may support verify operations on normal disk devices.
The SIOC_WRITE_WOE ioctl controls the write mode used for magnetooptical
disk devices. Normally magneto-optical write operations
require two physical head passes. The first pass erases the media
area to be written. The second pass actually writes the data.
Write-without-erase mode dramatically increases write performance by
skipping the first (erase media area) pass. To ensure that the
correct data results, it is essential that write-without-erase
operations be performed only on media that is known to be blank
(previously erased or never used). The SIOC_WRITE_WOE ioctl can be
used to enable or disable write-without-erase. A zero value disables
write-without-erase. The value 1 enables write-without-erase.
The SIOC_ERASE ioctl allows media areas to be explicitly erased. The
media area to be erased is specified via the start_lba and block_cnt
fields. Media areas erased in this manner can be written using
write-without-erase mode. Note that an erased media area is different
from a media area written with some data values (e.g. zeros). An
erased media area should not be read. Attempting to read an erased
media area generally results in an I/O error.
The SIOC_VERIFY_BLANK ioctl verifies that a media area has been erased
and is suitable for being written using write-without-erase mode. The
media area to be verified is specified via the start_lba and block_cnt
fields.
The following optical disk device specific information is included
from <sys/scsi.h>:
#define SIOC_WRITE_WOE _IOW('S', 17, int)
#define SIOC_VERIFY_WRITES _IOW('S', 18, int)
#define SIOC_ERASE _IOW('S', 19, struct scsi_erase)
#define SIOC_VERIFY_BLANK _IOW('S', 20, struct scsi_verify)
#define SIOC_VERIFY _IOW('S', 21, struct scsi_verify)
/* structure for SIOC_ERASE ioctl */
struct scsi_erase {
unsigned int start_lba;
unsigned short block_cnt;
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};
/* structure for SIOC_VERIFY_BLANK and SIOC_VERIFY ioctls */
struct scsi_verify {
unsigned int start_lba;
unsigned short block_cnt;
};
FILES [Toc] [Back]
/usr/include/sys/scsi.h
SEE ALSO [Toc] [Back]
mediainit(1), mknod(1M), ioctl(2), disk(7), scsi(7).
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