sg_read(8) - man - phpman

SG_READ(8)                                    SG3_UTILS                                   SG_READ(8)



NAME
       sg_read - read multiple blocks of data, optionally with SCSI READ commands

SYNOPSIS
       sg_read  [blk_sgio=0|1]  [bpt=BPT] [bs=BS] [cdbsz=6|10|12|16] count=COUNT [dio=0|1] [dpo=0|1]
       [fua=0|1] if=IFILE [mmap=0|1] [no_dxfer=0|1] [odir=0|1] [skip=SKIP] [time=TI]  [verbose=VERB]
       [--help] [--version]

DESCRIPTION
       Read  data  from  a Linux SCSI generic (sg) device, a block device or a normal file with each
       read command issued to the same offset or logical block address (lba). This can  be  used  to
       test  (or  time) disk caching, SCSI (or some other) transport throughput, and/or SCSI command
       overhead.

       When the COUNT value is positive, then up to BPT blocks are read at a time, until  the  COUNT
       is  exhausted. Each read operation starts at the same lba which, if SKIP is not given, is the
       beginning of the file or device.

       The COUNT value may be negative when IFILE  is  a  sg  device  or  is  a  block  device  with
       'blk_sgio=1'  set.  Alternatively  'bpt=0'  may be given. In these cases |COUNT| "zero block"
       SCSI READ commands are issued. "Zero block" means "do nothing" for SCSI READ 10,  12  and  16
       byte  commands  (but not for the 6 byte variant). In practice "zero block" SCSI READ commands
       have low latency and so are one way to measure SCSI command overhead.

       Please note: this is a very old utility that uses 32 bit  integers  for  disk  LBAs  and  the
       count.  Hence it will not be able to address beyond 2 Terabytes on a disk with logical blocks
       that are 512 bytes long.  Alternatives are the sg_dd and ddpt utilities.

OPTIONS
       blk_sgio=0 | 1
              The default action of this utility is to use the Unix read() command when the IFILE is
              a  block  device. In lk 2.6 many block devices can handle SCSI commands issued via the
              SG_IO ioctl. So when this option is set the SG_IO ioctl sends SCSI  READ  commands  to
              IFILE if it is a block device.

       bpt=BPT
              where  BPT  is the maximum number of blocks each read operation fetches.  Fewer blocks
              will be fetched when the remaining COUNT is less than BPT. The default value  for  BPT
              is 128. Note that each read operation starts at the same lba (as given by skip=SKIP or
              0).  If 'bpt=0' then the COUNT is interpreted as the number of zero  block  SCSI  READ
              commands to issue.

       bs=BS  where BS is the size (in bytes) of each block read. This must be the block size of the
              physical device (defaults to 512) if SCSI commands are being issued to IFILE.

       cdbsz=6 | 10 | 12 | 16
              size of SCSI READ commands issued on sg device names, or block devices if 'blk_sgio=1'
              is given. Default is 10 byte SCSI READ cdbs.

       count=COUNT
              when  COUNT  is a positive number, read that number of blocks, typically with multiple
              read operations. When COUNT is negative then |COUNT| SCSI READ commands are  performed
              requesting zero blocks to be transferred. This option is mandatory.

       dio=0 | 1
              default  is  0  which selects indirect IO. Value of 1 attempts direct IO which, if not
              available, falls back to indirect IO and notes this at completion. This option is only
              active if IFILE is an sg device.  If direct IO is selected and /proc/scsi/sg/allow_dio
              has the value of 0 then a warning is issued (and indirect IO is performed)

       dpo=0 | 1
              when set the disable page out (DPO) bit in SCSI READ commands is set.   Otherwise  the
              DPO bit is cleared (default).

       fua=0 | 1
              when  set the force unit access (FUA) bit in SCSI READ commands is set.  Otherwise the
              FUA bit is cleared (default).

       if=IFILE
              read from this IFILE. This argument must be given. If the IFILE is a normal file  then
              it  must  be seekable (if (COUNT > BPT) or skip=SKIP is given). Hence stdin is not ac‐
              ceptable (and giving "-" as the IFILE argument is reported as an error).

       mmap=0 | 1
              default is 0 which selects indirect IO. Value of 1 causes memory mapped IO to be  per‐
              formed.  Selecting  both dio and mmap is an error. This option is only active if IFILE
              is an sg device.

       no_dxfer=0 | 1
              when set then DMA transfers from the device are made into kernel buffers but  no  fur‐
              ther  (i.e.  there  is  no second copy into the user space). The default value is 0 in
              which case transfers are made into the user space.  When neither mmap nor dio  is  set
              then  data  transfer  are  copied  via kernel buffers (i.e. a double copy). Mainly for
              testing.

       odir=0 | 1
              when set opens an IFILE which is a block device with an additional O_DIRECT flag.  The
              default value is 0 (i.e. don't open block devices O_DIRECT).

       skip=SKIP
              all  read  operations  will start offset by SKIP bs-sized blocks from the start of the
              input file (or device).

       time=TI
              When TI is 0 (default) doesn't perform  timing.   When  1,  times  transfer  and  does
              throughput calculation, starting at the first issued command until completion. When 2,
              times transfer and does throughput calculation, starting at the second issued  command
              until  completion. When 3 times from third command, etc. An average number of commands
              (SCSI READs or Unix read()s) executed per second is also output.

       verbose=VERB
              as VERB increases so does the amount of debug output sent to stderr.  Default value is
              zero  which yields the minimum amount of debug output.  A value of 1 reports extra in‐
              formation that is not repetitive.

       --help Output the usage message then exit.

       --version
              Output the version string then exit.

NOTES
       Various numeric arguments (e.g. SKIP) may include multiplicative  suffixes  or  be  given  in
       hexadecimal. See the "NUMERIC ARGUMENTS" section in the sg3_utils(8) man page.

       Data  usually  gets  to the user space in a 2 stage process: first the SCSI adapter DMAs into
       kernel buffers and then the sg driver copies this data into user memory.  This is called "in‐
       direct  IO"  and  there  is a "dio" option to select "direct IO" which will DMA directly into
       user memory. Due to some issues "direct IO" is disabled in the sg driver and needs a configu‐
       ration change to activate it. This is typically done with "echo 1 > /proc/scsi/sg/allow_dio".
       An alternate way to avoid the 2 stage copy is to select memory mapped IO with 'mmap=1'.

SIGNALS
       The signal handling has been borrowed from dd: SIGINT, SIGQUIT and SIGPIPE output the  number
       of  remaining  blocks to be transferred; then they have their default action.  SIGUSR1 causes
       the same information to be output yet the copy continues.  All output caused  by  signals  is
       sent to stderr.

EXAMPLES
       Let us assume that /dev/sg0 is a disk and we wish to time the disk's cache performance.

          sg_read if=/dev/sg0 bs=512 count=1MB mmap=1 time=2

       This  command  will continually read 128  512 byte blocks from block 0.  The "128" is the de‐
       fault value for 'bpt' while "block 0" is chosen because the 'skip' argument  was  not  given.
       This  will  continue  until 1,000,000 blocks are read. The idea behind using 'time=2' is that
       the first 64 KiB read operation will involve reading the magnetic media while  the  remaining
       read operations will "hit" the disk's cache. The output of third command will look like this:

         time from second command to end was 4.50 secs, 113.70 MB/sec
         Average number of READ commands per second was 1735.27
         1000000+0 records in, SCSI commands issued: 7813

EXIT STATUS
       The  exit  status  of  sg_read is 0 when it is successful. 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 © 2000-2019 Douglas Gilbert
       This software is distributed under the GPL version 2. There is NO warranty; not even for MER‐
       CHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

SEE ALSO
       To  time streaming media read or write time see sg_dd is in the sg3_utils package and ddpt in
       a package of the same name.  The lmbench package contains lmdd  which  is  also  interesting.
       raw(8), dd(1)



sg3_utils-1.45                             September 2019                                 SG_READ(8)