Re: [Bug 10846] New: Slow write on LSISAS1068E (SAS6/iR) on kernel >= 2.6.22

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Mon, 2008-06-02 at 07:57 -0700, bugme-daemon@bugzilla.kernel.org
wrote:
> Very easy: We also check for an untar of the kernel (5x slower)
> debian-test:~# dd if=/dev/zero of=/root/test.cdrom bs=10k count=60000
> 60000+0 enregistrements lus
> 60000+0 enregistrements écrits
> 614400000 octets (614 MB) copiés, 1,17602 seconde, 522 MB/s
> debian-test:~# uname -a
> Linux debian-test.pr.univmed.fr 2.6.19.7 #1 SMP Fri May 30 15:55:50
> CEST 2008
> i686 GNU/Linux
> 
> debian:~# dd if=/dev/zero of=test.cdrom  bs=10k count=60000
> 60000+0 records in
> 60000+0 records out
> 614400000 bytes (614 MB) copied, 7.51225 seconds, 81.8 MB/s
> debian:~# uname -a
> Linux debian 2.6.22-4-vserver-686 #1 SMP Tue Feb 12 22:02:04 UTC 2008
> i686
> GNU/Linux

What exactly is test.cdrom on?  A single disk?  If so, then your first
result is definitely bogus.  The maximum bandwidth of a single SAS link
is 300 MB/s, so it looks like the kernel is either caching the write or
has timing issues.  For a direct to platter speed, the data sheet for
your drive gives 55-89MB/s, so I'd say 81.8 is well within the range.

James


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