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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] [--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 (de-
scriptor) 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 con-
figure 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 re-
ported 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 gettimeof-
day() 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 an-
other 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 de-
scription 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 de-
scription.
-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). Simi-
lar to the --size=OVERALL option in the main description but the units are differ-
ent.
-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 pres-
ence of this environment variable is equivalent to using --old (or -O) as the first com-
mand line option.
AUTHOR
Written by Douglas Gilbert
REPORTING BUGS
Report bugs to <dgilbert at interlog dot com>.
COPYRIGHT
Copyright (C) 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.
SEE ALSO
sg_read_buffer, sg_write_buffer, sg_test_rwbuf(all in sg3_utils)
sg3_utils-1.43 October 2017 SG_RBUF(8)
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