SG_RBUF(8)							     SG3_UTILS								SG_RBUF(8)

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  elimi-
	      nate 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" sec-
       tion 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 utiliza-
       tion 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 Doug Gilbert

REPORTING BUGS

       Report bugs to <dgilbert at interlog dot com>.

COPYRIGHT

       Copyright (C) 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  PUR-
       POSE.

SEE ALSO

       sg_read_buffer, sg_write_buffer, sg_test_rwbuf(all in sg3_utils)

sg3_utils-1.23							   January 2007 							SG_RBUF(8)