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SG_VERIFY(8) SG3_UTILS SG_VERIFY(8)
NAME
sg_verify - invoke SCSI VERIFY command(s) on a block device
SYNOPSIS
sg_verify [--0] [--16] [--bpc=BPC] [--count=COUNT] [--dpo] [--ff] [--ebytchk=BCH]
[--group=GN] [--help] [--in=IF] [--lba=LBA] [--ndo=NDO] [--quiet] [--readonly] [--verbose]
[--version] [--vrprotect=VRP] DEVICE
DESCRIPTION
Sends one or more SCSI VERIFY (10 or 16) commands to DEVICE. These SCSI commands are de-
fined in the SBC-2 and SBC-3 standards at http://www.t10.org and SBC-4 drafts.
When --ndo=NDO is not given then the verify starts at the logical block address given by
the --lba=LBA option and continues for --count=COUNT blocks. No more than --bpc=BPC blocks
are verified by each VERIFY command so if necessary multiple VERIFY commands are sent.
Medium verification operations are performed by the DEVICE (e.g. assuming each block has
additional EEC data, check this against the logical block contents). No news is good news
(i.e. if there are no verify errors detected then no messages are sent to stderr and the
Unix exit status is 0).
When --ndo=NDO is given then the --bpc=BPC option is ignored. A single VERIFY command is
issued and a comparison starts at the logical block address given by the --lba=LBA option
and continues for --count=COUNT blocks. The VERIFY command has an associated data-out buf-
fer that is NDO bytes long. The contents of the data-out buffer are obtained from the FN
file (if --in=FN is given) or from stdin. A comparison takes place between data-out buf-
fer and the logical blocks on the DEVICE. If the comparison is good then no messages are
sent to stderr and the Unix exit status is 0. If the comparison fails then a sense buffer
with a sense key of MISCOMPARE is returned; in this case the Unix exit status will be 14.
Messages will be sent to stderr associated with MISCOMPARE sense buffer unless the --quiet
option is given.
In SBC-3 revision 34 the BYTCHK field in all SCSI VERIFY commands was expanded from one to
two bits. That required some changes in the options of this utility, see the section below
on OPTION CHANGES.
OPTIONS
Arguments to long options are mandatory for short options as well. The options are ar-
ranged in alphabetical order based on the long option name.
-0, --0
a buffer NDO bytes long full of zeros is sent as the data-out part of a VERIFY com-
mand. So stdin is not read and if --in=IF is given, an error is generated. Useful
when BCH is 3 to check if some or all of DEVICE (e.g. a disk) is zero filled
blocks.
-S, --16
uses a VERIFY(16) command (default VERIFY(10)). Even without this option, using an
--lba=LBA which is too large, will cause the utility to issue a VERIFY(16) command.
-b, --bpc=BPC
this option is ignored if --ndo=NDO is given. Otherwise BPC specifies the maximum
number of blocks that will be verified by a single SCSI VERIFY command. The default
value is 128 blocks which equates to 64 KB for a disk with 512 byte blocks. If BPC
is less than COUNT then multiple SCSI VERIFY commands are sent to the DEVICE. For
the default VERIFY(10) BPC cannot exceed 0xffff (65,535) while for VERIFY(16) BPC
cannot exceed 0x7fffffff (2,147,483,647). For recent block devices (disks) this
value may be constrained by the maximum transfer length field in the block limits
VPD page.
-c, --count=COUNT
where COUNT specifies the number of blocks to verify. The default value is 1 . If
COUNT is greater than BPC (or its default value of 128) and NDO is not given, 0 or
less than multiple SCSI VERIFY commands are sent to the device. Otherwise COUNT be-
comes the contents of the verification length field of the SCSI VERIFY command is-
sued. The sg_readcap utility can be used to find the maximum number of blocks that
a block device (e.g. a disk) has.
-d, --dpo
disable page out changes the cache retention priority of blocks read on the de-
vice's cache to the lowest priority. This means that blocks read by other commands
are more likely to remain in the device's cache.
-E, --ebytchk=BCH
sets the BYTCHK field to BCH overriding the value (1) set by the --ndo=NDO option.
Values of 1, 2 or 3 are accepted for BCH however sbc3r34 reserves the value 2. If
this option is given then --ndo=NDO must also be given. If BCH is 3 then NDO should
be the size of one logical block (plus the size of some or all of the protection
information if VRP is greater than 0).
-f, --ff
a buffer NDO bytes long full of 0xff bytes is sent as the data-out part of a VERIFY
command. So stdin is not read and if --in=IF is given, an error is generated. Use-
ful when BCH is 3 to check if some or all of DEVICE (e.g. a disk) is 0xff byte
filled blocks.
-g, --group=GN
where GN becomes the contents of the group number field in the SCSI VERIFY(16) com-
mand. It can be from 0 to 63 inclusive. The default value for GN is 0. Note that
this option is ignored for the SCSI VERIFY(10) command.
-h, --help
output the usage message then exit.
-i, --in=IF
where IF is the name of a file from which NDO bytes will be read and placed in the
data-out buffer. This is only done when the --ndo=NDO option is given. If this op-
tion is not given then stdin is read. If IF is "-" then stdin is also used.
-l, --lba=LBA
where LBA specifies the logical block address of the first block to start the ver-
ify operation. LBA is assumed to be decimal unless prefixed by '0x' or a trailing
'h' (see below). The default value is 0 (i.e. the start of the device).
-n, --ndo=NDO
NDO is the number of bytes to obtain from the FN file (if --in=FN is given) or from
stdin. Those bytes are placed in the data-out buffer associated with the SCSI VER-
IFY command and NDO is placed in the verification length field in the cdb. The de-
fault value for NDO is 0 and the maximum value is dependent on the OS. If the
--ebytchk=BCH option is not given then the BYTCHK field in the cdb is set to 1.
-q, --quiet
suppress the sense buffer messages associated with a MISCOMPARE sense key that
would otherwise be sent to stderr. Still set the exit status to 14 which is the
sense key value indicating a MISCOMPARE .
-r, --readonly
opens the DEVICE read-only rather than read-write which is the default. The Linux
sg driver needs read-write access for the SCSI VERIFY command but other access
methods may require read-only access.
-v, --verbose
increase the level of verbosity, (i.e. debug output).
-V, --version
print the version string and then exit.
-P, --vrprotect=VRP
where VRP is the value in the vrprotect field in the VERIFY command cdb. It must be
a value between 0 and 7 inclusive. The default value is zero.
BYTCHK
BYTCHK is the name of a field (two bits wide) in the VERIFY(10) and VERIFY(16) commands.
When set to 1 or 3 (sbc3r34 reserves the value 2) it indicates that associated with the
SCSI VERIFY command, a data-out buffer will be sent for the device (disk) to check. Using
the --ndo=NDO option sets the BYTCHK field to 1 and NDO is the number of bytes placed in
the data-out buffer. Those bytes are obtained from stdin or IF (from the --in=FN option).
The --ebytchk=BCH option may be used to override the BYTCHK field value of 1 with BCH.
The calculation of NDO is left up to the user. Its value depends on the logical block size
(which can be found with the sg_readcap utility), the COUNT and the VRP values. If the VRP
is greater than 0 then each logical block will contain an extra 8 bytes (at least) of pro-
tection information.
When the BYTCHK field is 0 then the verification process done by the device (disk) is ven-
dor specific. It typically involves checking each block on the disk against its error cor-
rection codes (ECC) which is additional data also held on the disk.
Many Operating Systems put limits on the maximum size of the data-out (and data-in) buf-
fer. For Linux at one time the limit was less than 1 MB but has been increased somewhat.
OPTION CHANGES
Earlier versions of this utility had a --bytchk=NDO option which set the BYTCHK bit and
set the cdb verification length field to NDO. The shorter form of that option was -B NDO.
For backward compatibility that option is still present but not documented. In its place
is the --ndo=NDO whose shorter form of -n NDO. --ndo=NDO sets the BYTCHK field to 1 un-
less that is overridden by the --ebytchk=BCH.
NOTES
Various numeric arguments (e.g. LBA) may include multiplicative suffixes or be given in
hexadecimal. See the "NUMERIC ARGUMENTS" section in the sg3_utils(8) man page.
The amount of error correction and the number of retries attempted before a block is con-
sidered defective are controlled in part by the Verify Error Recovery mode page. A note in
the SBC-3 draft (rev 29 section 6.4.9 on the Verify Error Recovery mode page) advises that
to minimize the number of checks (and hence have the most "sensitive" verify check) do the
following in that mode page: set the EER bit to 0, the PER bit to 1, the DTE bit to 1, the
DCR bit to 1, the verify retry count to 0 and the verify recovery time limit to 0. Mode
pages can be modified with the sdparm utility.
The SCSI VERIFY(6) command defined in the SSC-2 standard and later (i.e. for tape drive
systems) is not supported by this utility.
EXIT STATUS
The exit status of sg_verify is 0 when it is successful. When BCH is other than 0 then a
comparison takes place and if it fails then the exit status is 14 which happens to be the
sense key value of MISCOMPARE. Otherwise see the EXIT STATUS section in the sg3_utils(8)
man page.
Earlier versions of this utility set an exit status of 98 when there was a MISCOMPARE.
AUTHORS
Written by Douglas Gilbert.
REPORTING BUGS
Report bugs to <dgilbert at interlog dot com>.
COPYRIGHT
Copyright (C) 2004-2019 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
sdparm(sdparm), sg_modes(sg3_utils), sg_readcap(sg3_utils), sg_inq(sg3_utils)
sg3_utils-1.45 December 2019 SG_VERIFY(8)
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