LeftHand Network RAID 5 space usage

[Larry Schuldt]

I am installing four P4500 G2 chassis with SANiQ 8.5. In creating thick provisioned LUNS (yes, I KNOW that HP recommends thin provisioning) the space allocated is double the LUN size. This is not how I would expect RAID 5 to work.

Any thoughts on this?

larry

[teledata]

Hardware RAID 5 is the way the physical disks are protected on a per-node basis.

Did you choose Network RAID 5 or Network RAID 0+1?

If you choose Network RAID 0+1 (or previous to SANiQ 8.5 this was called 2-way replication) then it will consume twice the space (to provide high availability).

The network RAID level is configured on a per volume basis, but the physical RAID is configured on a per NODE basis, and cannot be changed without re-striping the cluster.

[Larry Schuldt]

This is thick-provisioned network RAID 5. Is this a bug? Why is double the LUN size being allocated for network RAID 5? This means that there is no storage efficiency vs. network raid 0+1(formerly known as replication level 2)!

[Robert Lennie]

http://blogs.hp.com/p4000/2010/02/12/network-raid-5-and-6-explained/

Highlights:
Network RAID5 is a Background Parity Based Encoding method that reclaims disk space based on snapshots that are taken. I'll refer to it as PBNR (parity based network raid)
The top level volume is still a Network RAID10 volume, and that is to maintain performance and availability.
PBNR does not run on the top level volume because of the performance impact of the read-modify-write algorithms for small writes
Over time snapshots are merged over time, and most of the data end up in there
The greater the change rate, the less space is 'reclaimed'
Make sure the default schedule that is setup for the snapshots is setup to off peak hours

[Larry Schuldt]

Robert,

Thanks... I've had more time to play with it since that post. At the time, RAID 5 was new and we were just seeing it in the field. Then, I didn't realize that everything was first created as RAID 10 and then restriped to RAID 5 according to a schedule. It was something of a shock to realize how space-hungry RAID 5 luns could be, esp. if you do a restore on a full LUN. The original RAID 5 LUN stays, then all the changes go to a RAID 10 LUN so it chews up a bit more than 3 times the storage of the raw LUN, 2x for the RAID 10 and about 1.2x for the RAID 5. Ouch!

[Larry Schuldt]

c

[Mark...]

For Robert...
Not able to access the link you posted.
Please can you post it in this thread?
Cheers,
Mark...