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,
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,
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.