Provided by: sg3-utils_1.42-2ubuntu1.18.04.2_amd64 
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)