Provided by: sg3-utils_1.42-2ubuntu1.18.04.2_amd64 
NAME
sg_read - read multiple blocks of data, optionally with SCSI READ commands
SYNOPSIS
sg_read [blk_sgio=0|1] [bpt=BPT] [bs=BS] [cdbsz=6|10|12|16] count=COUNT [dio=0|1] [dpo=0|1] [fua=0|1]
if=IFILE [mmap=0|1] [no_dxfer=0|1] [odir=0|1] [skip=SKIP] [time=TI] [verbose=VERB] [--help] [--version]
DESCRIPTION
Read data from a Linux SCSI generic (sg) device, a block device or a normal file with each read command
issued to the same offset or logical block address (lba). This can be used to test (or time) disk
caching, SCSI (or some other) transport throughput, and/or SCSI command overhead.
When the COUNT value is positive, then up to BPT blocks are read at a time, until the COUNT is exhausted.
Each read operation starts at the same lba which, if SKIP is not given, is the beginning of the file or
device.
The COUNT value may be negative when IFILE is a sg device or is a block device with 'blk_sgio=1' set.
Alternatively 'bpt=0' may be given. In these cases |COUNT| "zero block" SCSI READ commands are issued.
"Zero block" means "do nothing" for SCSI READ 10, 12 and 16 byte commands (but not for the 6 byte
variant). In practice "zero block" SCSI READ commands have low latency and so are one way to measure SCSI
command overhead.
OPTIONS
blk_sgio=0 | 1
The default action of this utility is to use the Unix read() command when the IFILE is a block
device. In lk 2.6 many block devices can handle SCSI commands issued via the SG_IO ioctl. So when
this option is set the SG_IO ioctl sends SCSI READ commands to IFILE if it is a block device.
bpt=BPT
where BPT is the maximum number of blocks each read operation fetches. Fewer blocks will be
fetched when the remaining COUNT is less than BPT. The default value for BPT is 128. Note that
each read operation starts at the same lba (as given by skip=SKIP or 0). If 'bpt=0' then the
COUNT is interpreted as the number of zero block SCSI READ commands to issue.
bs=BS where BS is the size (in bytes) of each block read. This must be the block size of the physical
device (defaults to 512) if SCSI commands are being issued to IFILE.
cdbsz=6 | 10 | 12 | 16
size of SCSI READ commands issued on sg device names, or block devices if 'blk_sgio=1' is given.
Default is 10 byte SCSI READ cdbs.
count=COUNT
when COUNT is a positive number, read that number of blocks, typically with multiple read
operations. When COUNT is negative then |COUNT| SCSI READ commands are performed requesting zero
blocks to be transferred. This option is mandatory.
dio=0 | 1
default is 0 which selects indirect IO. Value of 1 attempts direct IO which, if not available,
falls back to indirect IO and notes this at completion. This option is only active if IFILE is an
sg device. If direct IO is selected and /proc/scsi/sg/allow_dio has the value of 0 then a warning
is issued (and indirect IO is performed)
dpo=0 | 1
when set the disable page out (DPO) bit in SCSI READ commands is set. Otherwise the DPO bit is
cleared (default).
fua=0 | 1
when set the force unit access (FUA) bit in SCSI READ commands is set. Otherwise the FUA bit is
cleared (default).
if=IFILE
read from this IFILE. This argument must be given. If the IFILE is a normal file then it must be
seekable (if (COUNT > BPT) or skip=SKIP is given). Hence stdin is not acceptable (and giving "-"
as the IFILE argument is reported as an error).
mmap=0 | 1
default is 0 which selects indirect IO. Value of 1 causes memory mapped IO to be performed.
Selecting both dio and mmap is an error. This option is only active if IFILE is an sg device.
no_dxfer=0 | 1
when set then DMA transfers from the device are made into kernel buffers but no further (i.e.
there is no second copy into the user space). The default value is 0 in which case transfers are
made into the user space. When neither mmap nor dio is set then data transfer are copied via
kernel buffers (i.e. a double copy). Mainly for testing.
odir=0 | 1
when set opens an IFILE which is a block device with an additional O_DIRECT flag. The default
value is 0 (i.e. don't open block devices O_DIRECT).
skip=SKIP
all read operations will start offset by SKIP bs-sized blocks from the start of the input file (or
device).
time=TI
When TI is 0 (default) doesn't perform timing. When 1, times transfer and does throughput
calculation, starting at the first issued command until completion. When 2, times transfer and
does throughput calculation, starting at the second issued command until completion. When 3 times
from third command, etc. An average number of commands (SCSI READs or Unix read()s) executed per
second is also output.
verbose=VERB
as VERB increases so does the amount of debug output sent to stderr. Default value is zero which
yields the minimum amount of debug output. A value of 1 reports extra information that is not
repetitive.
--help Output the usage message then exit.
--version
Output the version string then exit.
NOTES
Various numeric arguments (e.g. SKIP) may include multiplicative suffixes or be given in hexadecimal. See
the "NUMERIC ARGUMENTS" section in the sg3_utils(8) man page.
Data usually gets to the user space in a 2 stage process: first the SCSI adapter DMAs into kernel buffers
and then the sg driver copies this data into user memory. This is called "indirect IO" and there is a
"dio" option to select "direct IO" which will DMA directly into user memory. Due to some issues "direct
IO" is disabled in the sg driver and needs a configuration change to activate it. This is typically done
with "echo 1 > /proc/scsi/sg/allow_dio". An alternate way to avoid the 2 stage copy is to select memory
mapped IO with 'mmap=1'.
SIGNALS
The signal handling has been borrowed from dd: SIGINT, SIGQUIT and SIGPIPE output the number of remaining
blocks to be transferred; then they have their default action. SIGUSR1 causes the same information to be
output yet the copy continues. All output caused by signals is sent to stderr.
EXAMPLES
Let us assume that /dev/sg0 is a disk and we wish to time the disk's cache performance.
sg_read if=/dev/sg0 bs=512 count=1MB mmap=1 time=2
This command will continually read 128 512 byte blocks from block 0. The "128" is the default value for
'bpt' while "block 0" is chosen because the 'skip' argument was not given. This will continue until
1,000,000 blocks are read. The idea behind using 'time=2' is that the first 64 KiB read operation will
involve reading the magnetic media while the remaining read operations will "hit" the disk's cache. The
output of third command will look like this:
time from second command to end was 4.50 secs, 113.70 MB/sec
Average number of READ commands per second was 1735.27
1000000+0 records in, SCSI commands issued: 7813
EXIT STATUS
The exit status of sg_read is 0 when it is successful. Otherwise see the sg3_utils(8) man page.
AUTHORS
Written by Douglas Gilbert.
REPORTING BUGS
Report bugs to <dgilbert at interlog dot com>.
COPYRIGHT
Copyright © 2000-2012 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
To time streaming media read or write time see sg_dd is in the sg3_utils package. The lmbench package
contains lmdd which is also interesting. raw(8), dd(1)