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       sg_read [OPTION]...


       Read data from a Linux SCSI generic  (sg)  device,  a  raw
       devices  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.

              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).

              this  must be the block size of the physical device
              (defaults to 512)

       cdbsz=6 | 10 | 12 | 16
              size of SCSI READ  commands  issued  on  sg  device
              names.  Default is 10 byte SCSI READ command blocks

              read this number of blocks. This argument  must  be

       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.
              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)

              read from this FILE. This argument must be given

       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

              all read operations will start offset by BLOCKS bs-
              sized  blocks  from  the  start  of  input (file or

       time=0 | 1 | 2
              When 0 (default) doesn't perform timing.   when  1,
              times  transfer  and  does  throughput calculation,
              starting at the first issued command until  comple­

       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 mem­
       ory mapped IO.


       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=1M 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
       KB 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:

         time from second command to end  was  8.51  secs,  60.19
         1000000+0 records in, SCSI commands issued: 7813


       Written by Doug Gilbert.


       Report bugs to <dgilbert@interlog.com>.


       Copyright © 2000-2002 Douglas Gilbert
       This  software  is  distributed  under  the GPL version 2.
       There is NO warranty; not even for MERCHANTABILITY or FIT­


       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)

sg3_utils-0.98            February 2002                SG_READ(8)
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