LUN and Volume Thin Provisioning
I already described thin provisioning of VMware NFS volumes some time ago here. Now I want to discuss thin provisioning of LUNs.
LUNs are different from VMFS on top of NFS implementation, because LUN is an additional container inside of NetApp FlexVol. So if you’re using FC, you need to thin provision both LUN and volume:
> lun set reservation “/vol/targetvol/targetlun” disable
> vol options “targetvol” guarantee none
In fact, you can make the LUN thin and the volume thick. Then storage space that’s not used by the LUN, is returned to the volume level. But in this case it cannot be used by other volumes as a shared pool of space.
As the best practice, NetApp now recommends to set Fractional Reserve and Snap Reserve for your volumes to 0%. Don’t forget about that, if you want to save more storage space:
> vol options “targetvol” fractional_reserve 0
> snap reserve “targetvol” 0
Disable snapshots if you don’t use them:
> snap sched “targetvol” 0
It’s easy as that. Now you don’t waste your space by reserving it ahead, but use it as a shared pool of resources. But make sure to monitor aggregate free space. If you starting to run out of storage, plan purchase of new disks in advance or redistribute data between other aggregates.
Safety Features
Disabling volume level, LUN and snapshot reservations helps you to save storage space. The drawback of this approach is that you don’t have any mechanisms in place to prevent volume out-of-space situations. If you enable snapshots on the volume and they consume all the volume space, the volume goes offline. Very undesirable consequence. NetApp has two features that can serve as safety net in thin-provisioned environments: autosize and snap autodelete.
Snap autodelete automatically removes old snapshots if there is no space left inside the volume. Autosize, on the other hand, allows the volume to automatically grow to the specified limit (+20% to the volume size by default) in specified increments (5% of the volume size by the default). You can also specify what to do first autosize or snapshot autodelete by using ‘try_first’ option.
> snap autodelete “targetvol” on
> vol autosize “targetvol” on
> vol options “targetvol” try_first volume_grow
SnapMirror Considerations
If you use SnapMirroring and switch on the autosize on the source volume, then the destination volume won’t grow automatically. And SnapMirror will break the relationship if it runs out of space on the smaller destination volume. The trick here is to make the destination volume as big as the autosize limit for the source volume and thin provision the destination volume. By doing that you won’t run out of space on destination even if the source volume grows to its maximum.
Further reading
TR-3965: NetApp Thin Provisioning Deployment and Implementation Guide Data ONTAP 8.1 7-Mode
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