Ubuntu Manpage: sg_rbuf - reads data using SCSI READ BUFFER command

Provided by: sg3-utils_1.46-1build1_amd64

NAME

       sg_rbuf - reads data using SCSI READ BUFFER command

SYNOPSIS

       sg_rbuf  [--buffer=EACH]  [--dio] [--help] [--mmap] [--quick] [--size=OVERALL] [--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.

       This is a Linux only utility and only works when DEVICE is an sg device (e.g. "/dev/sg1").
       The  sg_read_buffer  utility  has  similar  functionality  and is ported to other OSes and
       within Linux can use bsg and normal block device names (e.g. "/dev/sdc").

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. Please use as first option.

-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 .
Since then this utility defaults to the newer command line options which can be overridden
by using --old (or -O) as the first option. See the ENVIRONMENT VARIABLES section for
another way to force the use of these older command line options.

-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, --new
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.

ENVIRONMENT VARIABLES

       Since  sg3_utils  version  1.23  the environment variable SG3_UTILS_OLD_OPTS can be given.
       When it is present this utility will  expect  the  older  command  line  options.  So  the
       presence  of  this  environment variable is equivalent to using --old (or -O) as the first
       command line option.

AUTHOR

       Written by Douglas Gilbert

REPORTING BUGS

       Report bugs to <dgilbert at interlog dot com>.

COPYRIGHT

       Copyright © 2000-2017 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.