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SG_SEEK(8) SG3_UTILS SG_SEEK(8)
NAME
sg_seek - send SCSI SEEK, PRE-FETCH(10) or PRE-FETCH(16) command
SYNOPSIS
sg_seek [--10] [--count=NC] [--grpnum=GN] [--help] [--immed] [--lba=LBA]
[--num-blocks=NUM] [--pre-fetch] [--readonly] [--skip=SB] [--time] [--verbose] [--version]
[--wrap-offset=WO] DEVICE
DESCRIPTION
Sends a SCSI SEEK(10), PRE-FETCH(10) or PRE-FETCH(16) command to the DEVICE. The SEEK com-
mand has been obsolete since SBC-2 (2005) but still is supported on some hard disks and
even some SSDs (solid state disks). The PRE-FETCH command can be viewed as SEEK's modern
replacement. Instead of talking about moving the disk heads to the track containing the
sort after LBA, it talks about bringing the sort after LBA (and a given number of blocks)
into the disk's cache. Also the PRE-FETCH commands have an IMMED field.
The PRE-FETCH commands can report "real" errors but usually they will report one of two
"good" statuses. To do this they return the rarely used CONDITION MET status. If the num-
ber of blocks does actually fit in the cache (when IMMED=0) or there is enough room in the
cache when the command arrives (when IMMED=1) then a CONDITION MET status is returned. If
the requested number of blocks did not fit (IMMED=0) or would not fit (IMMED=1) then sta-
tus GOOD is returned. So if a disk has a large cache and PRE-FETCH is used sparingly then
the command is more likely to return CONDITION MET than GOOD. This presents some SCSI
sub-systems with problems as due to its rareness they mishandle CONDITION MET and treat it
as an error (see NOTES section below).
OPTIONS
Arguments to long options are mandatory for short options as well.
-T, --10
use a 10 byte cdb command, either SEEK(10) or PRE-FETCH(10) command. In the absence
of the --pre-fetch option, the SEEK(10) command is used. If the --pre-fetch option
is given without this option then a PRE-FETCH(16) command is used.
-c, --count=NC
NC is the number of commands (one of SEEK(10), PRE-FETCH(10) or PRE-FETCH(16)) that
will be executed. The default value is 1. If an error occurs it is noted and the
program continues until NC is exhausted. If NC is 0 then options are checked and
the DEVICE is opened but no commands are sent.
-g, --grpnum=GN
GN is the group number, a value between 0 and 63 (in hex: 0x3f). The default value
is 0. This option is ignored if the selected command is SEEK(10).
-h, --help
output the usage message then exit.
-i, --immed
this option only applies to PRE-FETCH(10) and PRE-FETCH(16), setting the IMMED bit.
Without this option, the DEVICE returns after it has completed transferring all, or
part of, the requested blocks into the cache. If this option is given the DEVICE
returns after it has done sanity checks on the cdb (e.g. making sure the LBA is
greater than the number of available blocks) and before it does the transfer into
the cache.
Note that even when this option is given, the return status from the PRE-FETCH com-
mands is still either CONDITION MET status (if the cache seems to have enough free
space for the transfer) or a GOOD status (if the cache does not seem to have enough
free space).
-l, --lba=LBA
LBA is the starting logical block address that is placed in the command descriptor
block (cdb) of the selected command. Note that the LBA field in SEEK(10) and
PRE-FETCH(10) is a 32 bit quantity, while with PRE-FETCH(16) it is a 64 bit quan-
tity. The default value is 0 .
-n, --num-blocks=NUM
NUM is the number of blocks, starting at and including LBA, to place in the DE-
VICE's cache. The SEEK(10) command does not use the NUM value. For PRE-FETCH(10)
NUM is a 16 bit quantity, while for PRE-FETCH(16) it is a 32 bit quantity. The de-
fault value is 1 . If NUM is 0 then the DEVICE will attempt to transfer all blocks
from the given LBA to the end of the medium.
-p, --pre-fetch
this option selects either PRE-FETCH(10) or PRE-FETCH(16) commands. With the --10
also given, the PRE-FETCH(10) command is selected; without that option
PRE-FETCH(16) is selected. The default (in the absence of this and other 'select-
ing' options) the SEEK(10) command is selected.
-r, --readonly
this option sets a 'read-only' flag when the underlying operating system opens the
given DEVICE. This may not work since operating systems can not easily determine
whether a pass-through is a logical read or write operation so they take a risk
averse stance and require read-write type DEVICE opens irrespective of what is per-
formed by the pass-through.
-s, --skip=SB
SB is the number of logical block addresses to skip, between repeated commands when
NC is greater than 1. The default value of SB is 1 . SB may be set to 0 so that all
NC PRE-FETCH commands use the same LBA.
-t, --time
if given the elapsed time to execute NC commands is recorded. This is printed out
before this utility exits. If NC is greater than 1 then the the "per command" time
is also printed.
-v, --verbose
increase the level of verbosity, (i.e. debug output).
-V, --version
print the version string and then exit.
-w, --wrap-offset=WO
WO is the number of blocks, relative to LBA, that when exceeded, set the next com-
mand's logical block address back to LBA. Whether this "reset-to-LBA" action occurs
depends on the values NC and SB.
NOTES
Prior to Linux kernel 4.17 the CONDITION MET status was logged as an error. Recent ver-
sions of FreeBSD handle the CONDITION MET status properly.
If either the --count=NC or --verbose option is given then a summary line like the follow-
ing is output:
Command count=5, number of condition_mets=3, number of goods=2
before the utility exits.
EXIT STATUS
The exit status of sg_seek is 0 (GOOD) or 25 (CONDITION_MET) when this utility is success-
ful. If multiple commands are executed (e.g. when NC is greater than 1) then the result of
the last executed SEEK or PRE-FETCH command sets the exit status. Otherwise see the
sg3_utils(8) man page.
AUTHORS
Written by Douglas Gilbert.
REPORTING BUGS
Report bugs to <dgilbert at interlog dot com>.
COPYRIGHT
Copyright (C) 2018 Douglas Gilbert
This software is distributed under a FreeBSD license. There is NO warranty; not even for
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
SEE ALSO
sg_vpd(sg3_utils); sdparm(sdparm)
sg3_utils-1.43 September 2018 SG_SEEK(8)
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