sg_read.8

.TH SG_READ "8" "March 2005" "sg3_utils-1.13" SG3_UTILS
.SH NAME
sg_read \- read blocks of data continually from same offset
.SH SYNOPSIS
.B sg_dd
[\fIblk_sgio=0|1\fR] [\fIbpt=<n>\fR] [\fIbs=<n>\fR] [\fIcdbsz=6|10|12|16\fR]
\fIcount=<n>\fR [\fIdio=0|1\fR] [\fIif=<ifile>\fR] [\fIodir=0|1\fR]
[\fIskip=<n>\fR] [\fItime=<n>\fR] [\fI--version\fR]
.SH DESCRIPTION
.\" Add any additional description here
.PP
Read data from a Linux SCSI generic (sg) device, a block device, a raw
device or a normal file with each read command issued to the same offset.
This will test (or time) disk caching and/or SCSI (or some other) bus 
throughput.
.TP
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.
.TP
bpt=BLOCKS
each read operation will be made using this number of blocks (or less if 
near the end of count). Default is 128. Note also that each read operation
starts at the same offset (as given by skip or 0).
.TP
bs=BYTES
this
.B must
be the block size of the physical device (defaults to 512) if SCSI commands
are being issued to <ifile>.
.TP
cdbsz=6 | 10 | 12 | 16
size of SCSI READ commands issued on sg device names.
Default is 10 byte SCSI READ cdbs
.TP
count=BLOCKS
read this number of blocks. This argument must be given
.TP
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 actice 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)
.TP
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 is given).
Hence stdin is not acceptable (and giving "-" as the <ifile> argument is
reported as an error).
.TP
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.
.TP
odir= 0 | 1
when set opens an <ifile> which is a block device with an additional
O_DIRECT flag. Active when blk_sgio is either 0 or 1. The default value
is 0 (i.e. don't open block devices O_DIRECT).
.TP
skip=BLOCKS
all read operations will start offset by BLOCKS bs-sized blocks 
from the start of input (file or device)
.TP
time=<n>
When 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.
.PP
The input file must be either a sg device, a raw device or a normal file.
A raw device must be bound to a block device prior to using sg_raw.
See
.B raw(8)
for more information about binding raw devices.
.PP
BYTES and BLOCKS may be followed by one of these multiplicative suffixes:
c C *1; w W *2; b B *512; k K KiB *1,024; KB *1,000; m M MiB *1,048,576;
MB *1,000,000; g G Gib *(2**30); GB *(10**9). Also a suffix of
the form "x<n>" multiplies the leading number by <n>. These multiplicative
suffixes are compatible with GNU's dd command (since 2002) which claims
compliance with SI and with IEC 60027-2.
.PP
Alternatively numerical values can be given in hexadecimal preceded by
either "0x" or "0X". When hex numbers are given multipliers cannot be
used.
.PP
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.
.SH EXAMPLES
.PP
Let us assume that /dev/sg0 is a disk and we wish to time the disk's
cache performance.
.PP
   sg_read if=/dev/sg0 bs=512 count=1MB mmap=1 time=2
.PP
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 thid command will look like this:
.PP
  time from second command to end was 8.51 secs, 60.19 MB/sec
.br
  1000000+0 records in, SCSI commands issued: 7813
.SH AUTHORS
Written by Doug Gilbert.
.SH "REPORTING BUGS"
Report bugs to <dgilbert at interlog dot com>.
.SH COPYRIGHT
Copyright \(co 2000-2002 Douglas Gilbert
.br
This software is distributed under the GPL version 2. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
.SH "SEE ALSO"
To time streaming media read or write time see
.B sg_dd
is in the sg3_utils package. The lmbench package contains
.B lmdd
which is also interesting.
.B raw(8), dd(1)