Re: amr Serious performance issues

[Thor Lancelot Simon <tls%panix.com@localhost>]

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 you *sure* Linux is not turning on write-back caching without telling
you so?  I have seen several Linux device drivers do this.

I don't believe there is such a thing as a MegaRAID without write cache.
There are MegaRAIDs without *battery backup for the cache*, and indeed
these should not be used in write-back mode but that doesn't prevent the
driver from turning it on.

Thor