# phpman > man > SG_VERIFY(8) [SG_VERIFY(8)](https://www.chedong.com/phpMan.php/man/SGVERIFY/8/markdown) SG3_UTILS [SG_VERIFY(8)](https://www.chedong.com/phpMan.php/man/SGVERIFY/8/markdown) ## 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 defined in the SBC-2 and SBC-3 standards at 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 is‐ sued 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 buffer 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 buffer 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 arranged 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)](https://www.chedong.com/phpMan.php/man/VERIFY/16/markdown) command (default [VERIFY(10)](https://www.chedong.com/phpMan.php/man/VERIFY/10/markdown)). Even without this option, using an _--lba=LBA_ which is too large, will cause the utility to issue a [VERIFY(16)](https://www.chedong.com/phpMan.php/man/VERIFY/16/markdown) command. ### -b --bpc this option is ignored if _--ndo=NDO_ is given. Otherwise _BPC_ specifies the maximum num‐ ber 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)](https://www.chedong.com/phpMan.php/man/VERIFY/10/markdown) _BPC_ cannot exceed 0xffff (65,535) while for [VERIFY(16)](https://www.chedong.com/phpMan.php/man/VERIFY/16/markdown) _BPC_ cannot exceed 0x7fffffff (2,147,483,647). For recent block devices (disks) this value may be con‐ strained by the maximum transfer length field in the block limits VPD page. ### -c --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 issued. 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 device'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 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. Useful when _BCH_ is 3 to check if some or all of _DEVICE_ (e.g. a disk) is 0xff byte filled blocks. ### -g --group where _GN_ becomes the contents of the group number field in the SCSI [VERIFY(16)](https://www.chedong.com/phpMan.php/man/VERIFY/16/markdown) 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)](https://www.chedong.com/phpMan.php/man/VERIFY/10/markdown) command. ### -h --help output the usage message then exit. ### -i --in 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 option is not given then stdin is read. If _IF_ is "-" then stdin is also used. ### -l --lba where _LBA_ specifies the logical block address of the first block to start the verify 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_ 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 VERIFY command and _NDO_ is placed in the verification length field in the cdb. The default 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 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)](https://www.chedong.com/phpMan.php/man/VERIFY/10/markdown) and [VERIFY(16)](https://www.chedong.com/phpMan.php/man/VERIFY/16/markdown) commands. When set to 1 or 3 (sbc3r34 reserves the value 2) it indicates that associated with the SCSI VER‐ IFY 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 protection information. When the BYTCHK field is 0 then the verification process done by the device (disk) is vendor specific. It typically involves checking each block on the disk against its error correction 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) buffer. 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 unless that is overridden by the _--ebytchk=BCH_. ## NOTES Various numeric arguments (e.g. _LBA_) may include multiplicative suffixes or be given in hexa‐ decimal. See the "NUMERIC ARGUMENTS" section in the [sg3_utils(8)](https://www.chedong.com/phpMan.php/man/sg3utils/8/markdown) man page. The amount of error correction and the number of retries attempted before a block is consid‐ ered 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 fol‐ lowing 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)](https://www.chedong.com/phpMan.php/man/VERIFY/6/markdown) command defined in the SSC-2 standard and later (i.e. for tape drive sys‐ tems) 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 com‐ parison 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)](https://www.chedong.com/phpMan.php/man/sg3utils/8/markdown) 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 . ## COPYRIGHT Copyright © 2004-2019 Douglas Gilbert This software is distributed under a FreeBSD license. There is NO warranty; not even for MER‐ CHANTABILITY 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)](https://www.chedong.com/phpMan.php/man/SGVERIFY/8/markdown)