raw devices

©[@ÄØÿ¿8þÿ¿Michael Govorun (simon@baydel.com)
Thu, 20 Dec 2001 12:22:33 -0000


I have been performing some tests on raw devices, the results of
which I do not understand. I have 4 FC busses, each with one
SCSI disk, connect to a Linux system running 2.4.16. I use the raw
command to bind /dev/raw1 - /dev/raw4 to each of the devices.
With one process per raw device, running large sequential reads, I
got a total throughput of 340 Megabytes per second. I also
observed 85% CPU idle. Following this I performed some more
tests and then returned to this one. This time the total had gone
down to 180 and there was no free CPU. I realized that the first
time I ran the tests, each of the disks that the raw devices were
mapped to were mounted. I then verified that this data was being
transferred along the FC bus using an analyzer while the devices
were mounted. Can anyone explain this to me ? I find it hard to
believe that the disk should be permitted to be mounted when
using raw device mappings. If the disks should not be mounted
why is there such a great performance difference ?

Many Thanks

Simon.
__________________________

Simon Haynes - Baydel
Phone : 44 (0) 1372 378811
Email : simon@baydel.com
__________________________
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