Re: amr Serious performance issues

[Eduardo Horvath <eeh%NetBSD.org@localhost>]

On Tue, 11 Jan 2011, Hans Rosenfeld wrote:

> On Tue, Jan 11, 2011 at 04:06:20PM +0100, Manuel Bouyer wrote:
> > On Tue, Jan 11, 2011 at 03:09:03PM +0100, Hans Rosenfeld wrote:
> > > Have you used a MegaRAID recently with 5.1?
> > > 
> > > I have a similar performance issue with an AMI MegaRAID SCSI 320 without
> > > write cache. On a RAID5 of 4 73GB disks I get about 6MB/s write speed.
> > > Until recently I believed that this just old hardware and lack of write
> > > cache. Then I found out that Linux does about 27MB/s on the same hardware
> > > and configuration.
> > 
> > Are partitions aligned in the same way ?
> 
> I don't remember whether I used any partitions at all. The machine is
> currently down and far away, so I can't check that right now. I think
> most of the time I tested with dd directly to the raw device, using
> block sizes 128k and 512k. Is a specially aligned partitioning supposed
> to make that much of a difference?

I hope you're not comparing dd performance to the raw device between Linux 
and anything else.  Linux doesn't actually have any raw devices.  
Everything goes to the buffer cache, hence any performance numbers you get 
that way are bogus.

Eduardo