30 TB RAID6 + XFS slow write performance

[Dave Chinner]

On Fri, Jul 22, 2011 at 01:05:14PM -0500, Stan Hoeppner wrote:
> On 7/22/2011 1:10 AM, Michael Monnerie wrote:
> 
> > Yes, I just wanted to know about the corner cases, and how XFS behaves. 
> > Actually, we're changing over to using NetApps, and with their WAFL 
> > anyway I should drop all su/sw usage and just use 4KB blocks.
> 
> I've never used a NetApp filer myself.  However, that said, I would
> assume that WAFL is only in play for NFS/CIFS transactions since WAFL is
> itself a filesystem.

Netapp's website is busted, so here's a cached link:

http://webcache.googleusercontent.com/search?q=cache:9DdO2a16hdIJ:blogs.netapp.com/extensible_netapp/2008/10/what-is-wafl--3.html+netapp+san+wafl&cd=1&hl=en&ct=clnk&source=www.google.com

"The point is that WAFL is the part of the code that provides the
'read or write from-disk' mechanisms to both NFS and CIFS and SAN.
The semantics of a how the blocks are accessed are provided by
higher level code not by WAFL, which means WAFL is not a file
system."

If you can be bothered trolling for that entire series of blog posts
in the google cache, it's probably a good idea so you can get a
basic understanding of what WAFL actually is.

> When exposing LUNs from the same filer to FC and iSCSI hosts I would
> assume the filer acts just as any other SAN controller would.

It has it's own quirks, just like any other FC attached RAID array...

> In this case I would think you'd probably still want to align your
> XFS filesystem to the underlying RAID stripe from which the LUN
> was carved.

Which actually matters very little when WAFL between the FS and the
disk because WAFL uses copy-on-write and stages all it's writes
through NVRAM and so you've got no idea what the alignment of any
given address in the filesystem maps to, anyway.

Cheers,

Dave.
-- 
Dave Chinner
david at fromorbit.com