SCSI disk speed inconsistency

SCSI disk speed inconsistency

[Karthik Sarangan]

I get a read speed of 69MBps from my SCSI disk and a write speed of 
49MBps from the same.
An IOMeter benchmark on the net says that 68MBps is the sustained speed 
for a write.

Could someone tell me how to increase my IO speed?

I have a Linux Kernel 2.6 in a RedHat Enterprise Linux WS 4 and I used 
the foll.
Read : sg_dd if=/dev/sdb  of=/dev/null bs=262144 count=1024 dio=1
Write: sg_dd if=/dev/zero of=/dev/sdb  bs=262144 count=1024 dio=1

Re: SCSI disk speed inconsistency

[Douglas Gilbert]

Karthik Sarangan wrote:
> I get a read speed of 69MBps from my SCSI disk and a write speed of
> 49MBps from the same.
> An IOMeter benchmark on the net says that 68MBps is the sustained speed
> for a write.
> 
> Could someone tell me how to increase my IO speed?
> 
> I have a Linux Kernel 2.6 in a RedHat Enterprise Linux WS 4 and I used
> the foll.
> Read : sg_dd if=/dev/sdb  of=/dev/null bs=262144 count=1024 dio=1
> Write: sg_dd if=/dev/zero of=/dev/sdb  bs=262144 count=1024 dio=1

Karthik,
For accurate timing with "sd" devices use "odir=1" rather
than "dio=1". "dio=1" only applies to sg devices so it has
no effect in your above example. [Perhaps I should add a
warning about this in sg_dd.]

Also with sg_dd you should use "bs=512" and read about the
"bpt=<n>" option in the man page. Also look at the "blk_sgio=0|1"
and the "time=1" option.


Doug Gilbert