Disk IO issues
mmcgrath at redhat.com
Wed Dec 31 23:27:18 UTC 2008
On Wed, 31 Dec 2008, Ricky Zhou wrote:
> On 2008-12-31 10:49:56 PM, Xavier Lamien wrote:
> > Could you perform an hdparm -tT on that disk ?
> Timing cached reads: 2668 MB in 2.00 seconds = 1336.06 MB/sec
> Timing buffered disk reads: 1024 MB in 3.01 seconds = 340.69 MB/sec
> > Also, output an strace against your cat & dd commands.
> > if my memory is good enough, cat use mmap() which is faster than
> > read() (which is used by dd)
> I just straced dd and cat, and it looks like cat is using a block size
> of 4096 bytes while dd is using 512 bytes. I *think* they were both
> just using read(). Perhaps dd if=/dev/sde of=/dev/null bs=4096 would be
> a better command to compare against.
Ok, that explains the difference between dd and cat. Now why is the rest
of it so bad.
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