[CentOS] 3Ware 9550SX-4LP Performance

[Steve Bergman]

On Sat, 2006-09-30 at 20:56 -0700, Kirk Bocek wrote:
> I know there are a few 3Ware fans here and I was hoping to find some help. I 
> just built a new server using a 3Ware 9550SX-4LP with four disks in raid 5. 
> The array is fully initialized but I'm not getting the write performance I was 
> hoping for -- only 40 to 45MB/Sec.
> 
 If you are satisfied with the data integrity guarantees of XFS, which
are weaker than what EXT3 gives you by default, you might want to mount
your EXT3 filesystem with the data=writeback option.

This turns off the extra guarantees that ext3 gives you.

And if you do use XFS, make absolutely certain you can trust your UPS
and that the system is configured to do a normal shutdown before the UPS
runs out.

I've heard too many horror stories about XFS filesystems destroyed by a
power blip.

Yes,  I'm aware that it says in their FAQ that this was a problem that
was fixed in 1.1.  But from what I'm hearing, it is perhaps better, but
not fixed.

Just to compare the guarantees:

Ext3 (data=journal):  After an unclean shutdown, Fileysystem and
metadata are guaranteed to be consistent.  If the application was told
that a data write occurred, it did. (Slowest writes.)

Ext3 (data=ordered): After a power outage, the filesystem and metadata
are guaranteed to be consistent.  The file contents are guaranteed not
to be garbage.  BUT, the data might not be the latest written. (Medium
speed writes.)

Ext3 (data=writeback): This is the level that most journaled filesystems
give you.  After an unclean shutdown, filesystem and metadata are
guaranteed to be consistent.  File can contain garbage. (Fastest
writes.)

XFS: Same as data=writeback for ext3, but due to the way XFS nulls out
data blocks, and also delays allocation, data loss is more likely.  

Someone more familiar with XFS might want to elaborate or correct my
sketchy description of the reasons for XFS's relative fragility.

Be advised that changing the data journaling option for a *root*
filesystem is more involved than just changing it in /etc/fstab.
For /home, /var, etc. you can just add it to fstab and it will work.

-Steve