1SG_READ(8)                         SG3_UTILS                        SG_READ(8)
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NAME

6       sg_read - read blocks of data continually from same offset
7

SYNOPSIS

9       sg_read [blk_sgio=0|1] [bpt=BPT] [bs=BS] [cdbsz=6|10|12|16] count=COUNT
10       [dio=0|1]  [dpo=0|1]  [fua=0|1]  if=IFILE   [mmap=0|1]   [no_dxfer=0|1]
11       [odir=0|1] [skip=SKIP] [time=TI] [verbose=VERB] [--help] [--version]
12

DESCRIPTION

14       Read  data  from  a Linux SCSI generic (sg) device, a block device or a
15       normal file with each read command issued to the same offset or logical
16       block  address  (lba). This can be used to test (or time) disk caching,
17       SCSI (or some other) transport throughput, and/or  SCSI  command  over‐
18       head.
19
20       When  the  COUNT value is positive, then up to BPT blocks are read at a
21       time, until the COUNT is exhausted. Each read operation starts  at  the
22       same  lba  which, if SKIP is not given, is the beginning of the file or
23       device.
24
25       The COUNT value may be negative when IFILE is a sg device or is a block
26       device  with  'blk_sgio=1'  set. Alternatively 'bpt=0' may be given. In
27       these cases |COUNT| "zero block" SCSI READ commands are  issued.  "Zero
28       block"  means  "do  nothing"  for SCSI READ 10, 12 and 16 byte commands
29       (but not for the 6 byte variant). In practice "zero  block"  SCSI  READ
30       commands  have  low  latency and so are one way to measure SCSI command
31       overhead.
32

OPTIONS

34       blk_sgio=0 | 1
35              The default action of this utility is to  use  the  Unix  read()
36              command  when  the IFILE is a block device. In lk 2.6 many block
37              devices can handle SCSI commands issued via the SG_IO ioctl.  So
38              when this option is set the SG_IO ioctl sends SCSI READ commands
39              to IFILE if it is a block device.
40
41       bpt=BPT
42              where BPT is the maximum number of blocks  each  read  operation
43              fetches.   Fewer blocks will be fetched when the remaining COUNT
44              is less than BPT. The default value for BPT is  128.  Note  that
45              each  read  operation  starts  at  the  same  lba  (as  given by
46              skip=SKIP or 0).  If 'bpt=0' then the COUNT  is  interpreted  as
47              the number of zero block SCSI READ commands to issue.
48
49       bs=BS  where BS is the size (in bytes) of each block read. This must be
50              the block size of the physical device (defaults to 512) if  SCSI
51              commands are being issued to IFILE.
52
53       cdbsz=6 | 10 | 12 | 16
54              size  of  SCSI READ commands issued on sg device names, or block
55              devices if 'blk_sgio=1' is given. Default is 10 byte  SCSI  READ
56              cdbs.
57
58       count=COUNT
59              when  COUNT  is  a  positive number, read that number of blocks,
60              typically with multiple read operations. When COUNT is  negative
61              then  |COUNT|  SCSI  READ commands are performed requesting zero
62              blocks to be transferred. This option is mandatory.
63
64       dio=0 | 1
65              default is 0 which selects indirect  IO.  Value  of  1  attempts
66              direct IO which, if not available, falls back to indirect IO and
67              notes this at completion. This option is only active if IFILE is
68              an    sg    device.     If    direct    IO   is   selected   and
69              /proc/scsi/sg/allow_dio has the value of 0  then  a  warning  is
70              issued (and indirect IO is performed)
71
72       dpo=0 | 1
73              when set the disable page out (DPO) bit in SCSI READ commands is
74              set.  Otherwise the DPO bit is cleared (default).
75
76       fua=0 | 1
77              when set the force unit access (FUA) bit in SCSI  READ  commands
78              is set.  Otherwise the FUA bit is cleared (default).
79
80       if=IFILE
81              read  from this IFILE. This argument must be given. If the IFILE
82              is a normal file then it must be seekable (if (COUNT >  BPT)  or
83              skip=SKIP  is  given). Hence stdin is not acceptable (and giving
84              "-" as the IFILE argument is reported as an error).
85
86       mmap=0 | 1
87              default is 0 which selects indirect IO. Value of 1 causes memory
88              mapped  IO  to  be  performed. Selecting both dio and mmap is an
89              error. This option is only active if IFILE is an sg device.
90
91       no_dxfer=0 | 1
92              when set then DMA transfers from the device are made into kernel
93              buffers  but  no  further (i.e. there is no second copy into the
94              user space). The default value is 0 in which case transfers  are
95              made into the user space.  When neither mmap nor dio is set then
96              data transfer are copied  via  kernel  buffers  (i.e.  a  double
97              copy). Mainly for testing.
98
99       odir=0 | 1
100              when  set  opens  an IFILE which is a block device with an addi‐
101              tional O_DIRECT flag. The default value is 0  (i.e.  don't  open
102              block devices O_DIRECT).
103
104       skip=SKIP
105              all  read  operations  will start offset by SKIP bs-sized blocks
106              from the start of the input file (or device).
107
108       time=TI
109              When TI is 0 (default) doesn't perform timing.   When  1,  times
110              transfer  and does throughput calculation, starting at the first
111              issued command until completion. When 2, times transfer and does
112              throughput  calculation,  starting  at the second issued command
113              until completion. When 3 times from third command, etc. An aver‐
114              age number of commands (SCSI READs or Unix read()s) executed per
115              second is also output.
116
117       verbose=VERB
118              as VERB increases so does the amount of  debug  output  sent  to
119              stderr.   Default  value is zero which yields the minimum amount
120              of debug output.  A value of 1 reports extra information that is
121              not repetitive.
122
123       --help Output the usage message then exit.
124
125       --version
126              Output the version string then exit.
127

NOTES

129       Various  numeric  arguments (e.g. SKIP) may include multiplicative suf‐
130       fixes or be given in hexadecimal. See the "NUMERIC  ARGUMENTS"  section
131       in the sg3_utils(8) man page.
132
133       Data  usually  gets  to  the user space in a 2 stage process: first the
134       SCSI adapter DMAs into kernel buffers and then  the  sg  driver  copies
135       this  data into user memory.  This is called "indirect IO" and there is
136       a "dio" option to select "direct IO" which will DMA directly into  user
137       memory. Due to some issues "direct IO" is disabled in the sg driver and
138       needs a configuration change to activate it.  This  is  typically  done
139       with  "echo 1 > /proc/scsi/sg/allow_dio". An alternate way to avoid the
140       2 stage copy is to select memory mapped IO with 'mmap=1'.
141

SIGNALS

143       The signal handling has been borrowed from dd: SIGINT, SIGQUIT and SIG‐
144       PIPE output the number of remaining blocks to be transferred; then they
145       have their default action.  SIGUSR1 causes the same information  to  be
146       output yet the copy continues.  All output caused by signals is sent to
147       stderr.
148

EXAMPLES

150       Let us assume that /dev/sg0 is a disk and we wish to  time  the  disk's
151       cache performance.
152
153          sg_read if=/dev/sg0 bs=512 count=1MB mmap=1 time=2
154
155       This  command  will continually read 128  512 byte blocks from block 0.
156       The "128" is the default value for 'bpt'  while  "block  0"  is  chosen
157       because  the  'skip'  argument  was not given. This will continue until
158       1,000,000 blocks are read. The idea behind using 'time=2' is  that  the
159       first  64  KiB  read  operation will involve reading the magnetic media
160       while the remaining read operations will "hit" the  disk's  cache.  The
161       output of third command will look like this:
162
163         time from second command to end was 4.50 secs, 113.70 MB/sec
164         Average number of READ commands per second was 1735.27
165         1000000+0 records in, SCSI commands issued: 7813
166

EXIT STATUS

168       The  exit  status  of sg_read is 0 when it is successful. Otherwise see
169       the sg3_utils(8) man page.
170

AUTHORS

172       Written by Doug Gilbert.
173

REPORTING BUGS

175       Report bugs to <dgilbert at interlog dot com>.
176
178       Copyright © 2000-2009 Douglas Gilbert
179       This software is distributed under the GPL version 2. There is NO  war‐
180       ranty;  not  even  for MERCHANTABILITY or FITNESS FOR A PARTICULAR PUR‐
181       POSE.
182

SEE ALSO

184       To time streaming media  read  or  write  time  see  sg_dd  is  in  the
185       sg3_utils  package.  The  lmbench  package  contains lmdd which is also
186       interesting.  raw(8), dd(1)
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189
190sg3_utils-1.27                    March 2009                        SG_READ(8)
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