P4800 RAID 10 and Thin Provisioning

[CMarty]

Hi All...

First time poster.   I have (8) Storage Systems in a P4800 G2 SAN config totaling 122 TB w\ Hardware RAID 5 underneith.  I have mostly full provisioned volumes. It's not clear to me how thin provisioning is affecting how much space I have or can free up. For example.

My full provisioned volumeX is 4TB.  Network Raid 10 (2-way mirror) reports 11.48TB. Which to me = 4TB*2 (because of the mirror) =8TB plus approx 3.48TB of snapshots. Am I correct so far? The cluster Volume Use summary is reporting that I can reclaim 8TB if I thinprovision it.

Can someone please explain that?

Thanks Much.

[oikjn · Accepted solution]

I"m not 100% sure how HP handles full provisioned volume snapshots so I can't say how those will change if you go to thin, but I would guess that the estimate numbers CMC says it can recover are accurate.

Thin provisioning will only allocate space on the san when data is written to the device.  In the case of NR10, every byte of data written will take up two bytes of space on the san.  If you have an 8TB LUN that only have 100GB data written to it, it would only take up 200GB space on the san for thin provisioned NR10.  That said, if you write a temp file that is 7TB on that 8TB volume, the volume would expand to take up 14TB on the san and unfortunately HP currently has no way to shrink that volume back down if you delete that file from the drive.

I use thin provisioning on my LUNs, but you have to be very careful to watch for LUN expansion and I would plan to assuming that your LUN will grow and at some point the only way to shrink that data is to migrate the data from the current LUN to a new LUN and delete the old.  Most of my LUNs are hyper-v csv volumes, so its no big deal to migrate all the data from a LUN to another one, but this would be much more of a headache if your LUN hosts your OS or something that isn't easily portable.