How does LHN Thin provisioning handle disk defragmentation

[Kelvin Teck Chiew Yong]

Hi, NetApps claim that LHN thin provisioning will cause disk to be fragmented, and the defragmentation for LHN is not efficient. I could not find related documents that explain on defragmentation of disks for LHN. Please advise, thanks.

[teledata]

Generally speaking disk fragmentation should not be a significant issue in a LeftHand topology.

Since your entire cluster is one large array, all disks are distributing I/O load for multiple iSCSI sessions, so the drives are already moving around "thrashing" to serve up multiple data streams, so weather the data is fragmented or sequential, the drive arms are still moving all over the place anyway. You are typically overcoming this by having more physical spindles to distribute the IO over more and more drives. So as your load increases, you add more modules.

I suspect you could show performance degredation from fragmentation if you compared single (read:dedicated) iSCSI session performing a large sequential data operation. AFAIK the Lefthand design is optimized more for random IO (than sequential/streaming IO) and you may see lower I/O due to the "fragmentation" for sequential/streaming type applications...

I'd like to hear the answer to this from the HP/LeftHand engineers, as this is just my outside observations...

[Gauche · Accepted solution]

They'll say anything :)

The HP LeftHand P4000 Thin Provisioning does not have any negative effect on disk fragmentation. Either from a host / OS / File system perspective nor internal to the SAN itself.

Entirely separate from thin provisioning the LeftHand SANs use a method internally to lay out volumes sequentially in what we call "strides" for optimal performance. Additionally there is a background task that will optimize these "strides" *if* they could be laid out better as needed. This background process is only really needed if a SAN is very full, so has less options to lay out strides optimally in the first place.

All of this is done automatically by the system and needs no administrator input or intervention.

[Mike Povall]

The other thing to remember is that if you do create a thin provisioned volume DO NOT defragment the volume as this will have the effect of turning it into a fully provisioned volume.

Mike.