NAME

       sg_rbuf - reads data using SCSI READ BUFFER command

SYNOPSIS

       sg_rbuf  [--buffer=EACH]  [--dio]  [--help]  [--mmap]  [--quick] [--size=OVERALL] [--test]
       [--verbose] [--version] DEVICE

       sg_rbuf [-b=EACH_KIB] [-d] [-m] [-q] [-s=OVERALL_MIB] [-t] [-v] [-V] DEVICE

DESCRIPTION

       This command reads data with the SCSI READ BUFFER command and then discards it.  Typically
       the  data being read is from a disk's memory cache. It is assumed that the data is sourced
       quickly (although this is not guaranteed by the SCSI standards) so that it is faster  than
       reading data from the media.  This command is designed for timing transfer speeds across a
       SCSI transport.

       To fetch the data with a SCSI READ  BUFFER  command  and  optionally  decode  it  see  the
       sg_read_buffer  utility.  There  is  also a sg_write_buffer utility useful for downloading
       firmware amongst other things.

       This utility supports two command line syntaxes, the preferred one is shown first  in  the
       synopsis  and  explained  in  this section. A later section 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, --buffer=EACH
              where EACH is the number of bytes to be transferred by each  READ  BUFFER  command.
              The  default  is  the  actual  available  buffer  size  returned by the READ BUFFER
              (descriptor) command. The maximum is the same as the default, hence  this  argument
              can  only  be  used  to  reduce the size of each transfer to less than the device's
              actual available buffer size.

       -d, --dio
              use direct IO if available. This option is only available if the  DEVICE  is  a  sg
              driver  device  node  (e.g.  /dev/sg1).  In this case the sg driver will attempt to
              configure the DMA from the SCSI adapter to transfer directly into user memory. This
              will  eliminate  the  copy  via  kernel buffers. If not available then this will be
              reported and indirect IO will be done instead.

       -h, --help
              print usage message then exit.

       -m, --mmap
              use memory mapped IO if available. This option is only available if the DEVICE is a
              sg  driver  device node (e.g. /dev/sg1). In this case the sg driver will attempt to
              configure the DMA from the SCSI adapter to transfer directly into user memory. This
              will eliminate the copy via kernel buffers.

       -O, --old
              switch to older style options.

       -q, --quick
              only  transfer the data into kernel buffers (typically by DMA from the SCSI adapter
              card) and do not move it into the user space. This option is only available if  the
              DEVICE is a sg driver device node (e.g. /dev/sg1).

       -s, --size=OVERALL
              where  OVERALL  is  the  size  of  total  transfer in bytes. The default is 200 MiB
              (200*1024*1024 bytes). The actual number of bytes transferred may be slightly  less
              than  requested  since  all transfers are the same size (and an integer division is
              involved rounding towards zero).

       -t, --time
              times the bulk data transfer component of this command. The elapsed time is printed
              out  plus  a  MB/sec  calculation.  In  this  case  "MB"  is  1,000,000  bytes. The
              gettimeofday() system call is used internally for the time calculation.

       -v, --verbose
              increase level of verbosity. Can be used multiple times.

       -V, --version
              print out version string then exit.

NOTES

       This command is typically used on modern SCSI disks which have a RAM cache in their  drive
       electronics. If no IO to the magnetic media, or slower devices like flash RAM, is involved
       then the disk may be able to source data fast enough to saturate the bandwidth of the SCSI
       transport. The bottleneck may then be the DMA element in the HBA, the Linux drivers or the
       host machine's hardware (e.g. speed of RAM).

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

EXAMPLES

       On  the test system /dev/sg0 corresponds to a fast disk on a U2W SCSI bus (max 80 MB/sec).
       The disk specifications state that its cache is 4 MB.
          $ time ./sg_rbuf /dev/sg0
       READ BUFFER reports: buffer capacity=3434944,
           offset boundary=6
       Read 200 MiB (actual 199 MiB, 209531584 bytes),
           buffer size=3354 KiB
       real 0m5.072s, user 0m0.000s, sys 0m2.280s

       So that is approximately 40 MB/sec at 40 % utilization. Now with the addition of the  "-q"
       option this throughput improves and the utilization drops to 0%.
          $ time ./sg_rbuf -q /dev/sg0
       READ BUFFER reports: buffer capacity=3434944,
           offset boundary=6
       Read 200 MiB (actual 199 MiB, 209531584 bytes),
           buffer size=3354 KiB
       real 0m2.784s, user 0m0.000s, sys 0m0.000s

EXIT STATUS

       The  exit status of sg_rbuf 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.

       -b=EACH_KIB
              where EACH_KIB is the number of Kilobytes (i.e. 1024 byte units) to be  transferred
              by  each  READ  BUFFER  command.  Similar  to  the --buffer=EACH option in the main
              description but the units are different.

       -d     use direct IO if available. Equivalent to the --dio option in the main description.

       -m     use memory mapped IO if available. Equivalent to the  --mmap  option  in  the  main
              description.

       -N     switch to the newer style options.

       -q     only  transfer the data into kernel buffers (typically by DMA from the SCSI adapter
              card) and do not move it into the user space.  Equivalent to the --quick option  in
              the main description.

       -s=OVERALL_MIB
              where  OVERALL_MIB  is  the  size  of  total transfer in Megabytes (1048576 bytes).
              Similar to the --size=OVERALL option in the main  description  but  the  units  are
              different.

       -t     times  the  bulk  data transfer component of this command. Equivalent to the --time
              option in the main description.

       -v     increase level of verbosity. Can be used multiple times.

       -V     print out version string then exit.

AUTHOR

       Written by Douglas Gilbert

REPORTING BUGS

       Report bugs to <dgilbert at interlog dot com>.

COPYRIGHT

       Copyright © 2000-2007 Douglas Gilbert
       This software is distributed under the GPL version 2. There is NO warranty; not  even  for
       MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

SEE ALSO

       sg_read_buffer, sg_write_buffer, sg_test_rwbuf(all in sg3_utils)