sg_readcap man page on Scientific
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SG_READCAP(8) SG3_UTILS SG_READCAP(8)
NAME
sg_readcap - sends a SCSI READ CAPACITY command
SYNOPSIS
sg_readcap [--16] [--brief] [--help] [--hex] [--lba=LBA] [--long]
[--pmi] [--raw] [--verbose] [--version] DEVICE
sg_readcap [-16] [-b] [-h] [-H] [-lba=LBA] [-pmi] [-r] [-v] [-V] DEVICE
DESCRIPTION
The normal action of the SCSI READ CAPACITY command is to fetch the
number of blocks (and block size) from the DEVICE.
The SCSI READ CAPACITY command (both 10 and 16 byte cdbs) actually
yield the block address of the last block and the block size. The num‐
ber of blocks is thus one plus the block address of the last block (as
blocks are counted origin zero (i.e. starting at block zero)). This is
the source of many "off by one" errors.
The READ CAPACITY(16) response provides additional information not
found in the READ CAPACITY(10) response. This includes protection and
thin provisioning information, plus the number of logical blocks per
physical block. So even though the media size may not exceed what READ
CAPACITY(10) can show, it may still be useful to examine the response
to READ CAPACITY(16). Sadly there are horrible SCSI command set imple‐
mentations in the wild that crash when the READ CAPACITY(16) command is
sent to them.
Device capacity is the product of the number of blocks by the block
size. This utility outputs this figure in bytes, MiB (1048576 bytes
per MiB) and GB (1000000000 bytes per GB).
If sg_readcap is called without the --long option then the 10 byte cdb
version (i.e. READ CAPACITY (10)) is sent to the DEVICE. If the number
of blocks in the response is reported as 0xffffffff (i.e. (2**32 - 1) )
and the --hex option has not been given, then READ CAPACITY (16) is
called and its response is output.
This utility supports two command line syntaxes, the preferred one is
shown first in the synopsis and explained in this section. A later sec‐
tion on the old command line syntax outlines the second group of
options.
OPTIONS
Arguments to long options are mandatory for short options as well.
-b, --brief
outputs two hex numbers (prefixed with '0x' and space separated)
to stdout. The first number is the maximum number of blocks on
the device (which is one plus the lba of the last accessible
block). The second number is the size in bytes of each block. If
the operation fails then "0x0 0x0" is written to stdout.
-h, --help
print out the usage message then exit.
-H, --hex
output the response to the READ CAPACITY command (either the 10
or 16 byte cdb variant) in ASCII hexadecimal on stdout.
-L, --lba=LBA
used in conjunction with --pmi option. This variant of READ
CAPACITY will yield the last block address after LBA prior to a
delay. For a disk, given a LBA it yields the highest numbered
block on the same cylinder (i.e. before the heads need to move).
LBA is assumed to be decimal unless prefixed by "0x" or it has a
trailing "h". Defaults to 0.
-l, --long
Use the 16 byte cdb variant of the READ CAPACITY command. The
default action is to use the 10 byte cdb variant which limits
the maximum block address to (2**32 - 2). When a 10 byte cdb
READ CAPACITY command is used on a device whose size is too
large then a last block address of 0xffffffff is returned (if
the device complies with SBC-2).
--16 Use the 16 byte cdb variant of the READ CAPACITY command. See
the '--long' option.
-O, --old
switch to older style options.
-p, --pmi
partial medium indicator: for finding the next block address
prior to some delay (e.g. head movement). In the absence of this
option, the total number of blocks and the block size of the
device are output. Used in conjunction with the --lba=LBA
option.
-r, --raw
output response in binary to stdout.
-v, --verbose
increase level of verbosity. Can be used multiple times.
-V, --version
outputs version string then exits.
NOTES
In the 2.4 series of Linux kernels the DEVICE must be a SCSI generic
(sg) device. In the 2.6 series block devices (e.g. SCSI disks and DVD
drives) can also be specified. For example "sg_readcap /dev/sda" and
"sg_readcap /dev/hdd" (if /dev/hdd is a ATAPI CD/DVD device) will work
in the 2.6 series kernels.
EXIT STATUS
The exit status of sg_readcap is 0 when it is successful. Otherwise see
the sg3_utils(8) man page.
OLDER COMMAND LINE OPTIONS
The options in this section were the only ones available prior to
sg3_utils version 1.23 . In sg3_utils version 1.23 and later these
older options can be selected by either setting the SG3_UTILS_OLD_OPTS
environment variable or using '--old' (or '-O) as the first option.
-16 Use the 16 byte cdb variant of the READ CAPACITY command.
Equivalent to --long in the main description.
-b utility outputs two hex numbers (prefixed with '0x' and space
separated) to stdout. The first number is the maximum number of
blocks on the device (which is one plus the lba of the last
accessible block). The second number is the size of each block.
If the operation fails then "0x0 0x0" is written to stdout.
Equivalent to --brief in the main description.
-h output the usage message then exit. Giving the -? option also
outputs the usage message then exits.
-H output the response to the READ CAPACITY command (either the 10
or 16 byte cdb variant) in ASCII hexadecimal on stdout.
-lba=LBA
used in conjunction with -pmi option. This variant of READ
CAPACITY will yield the last block address after LBA prior to a
delay. Equivalent to --lba=LBA in the main description.
-N switch to the newer style options.
-pmi partial medium indicator: for finding the next block address
prior to some delay (e.g. head movement). In the absence of this
switch, the total number of blocks and the block size of the
device are output. Equivalent to --pmi in the main description.
-r output response in binary (to stdout).
-v verbose: print out cdb of issued commands prior to execution.
'-vv' and '-vvv' are also accepted yielding greater verbosity.
-V outputs version string then exits.
AUTHORS
Written by Douglas Gilbert
COPYRIGHT
Copyright © 1999-2009 Douglas Gilbert
This software is distributed under the GPL version 2. There is NO war‐
ranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PUR‐
POSE.
SEE ALSO
sg_inq(sg3_utils)
sg3_utils-1.28 September 2009 SG_READCAP(8)
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