Disk IO issues

[Mike McGrath]

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 ?
> /dev/sde:
>  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.

	-Mike