Space savings of Vraid5 vs Vraid1

[Chris Rasmussen [FAU]]

Uwe, finally you are here haha.

So are you saying that unless *all* the disks in the group are the same size there's *no* way of calculating the difference in DG space used by Vraid1 Vraid5?

[TTr]

I would use 25% instead of 20%, i.e.
500GB Vraid1 = 1GB used DG space
500GB Vraid5 = 625GB used DG space <====
100GB Vraid1 = 200GB used DG space
100GB Vraid5 = 125GB used DG space <====

Going back to the very first line in this thread, a 1.2TB Vraid1 would use 2.4TB spindle space and if you converted that to Vraid5 you would get 1.92TB, a .72TB gain of usable space.

[Uwe Zessin]

Agree: if you want to calculate the 'raw space' (=data+parity) used by VRAID-5 you have to multiply by 1.25 (if you pretend it has 25% overhead) or divide by 0.8 (if you pretend it has 20% overhead)...


Chris, here is an easy example:

VRAID-1 data can only be stored on a pair of disk drives. Create a disk group with an odd number of disk drives and fill it with VRAID-1 -- one of the disks stays empty, because it has no partner where a mirror copy can be stored.

This mechanisms is deeply engraved in the data layout.

Now fill the same disk group with VRAID-5. You will see that all disk drives will be filled with data, because VRAID-5 has a different allocation strategy.

You have won an additional disk drive's capacity.

[Chris Rasmussen [FAU]]

Well, yeah. D+P is really all I'm interested in as the parity info is essentially disk space when you're talking about how much 'space' a vdisk uses. If you don't take parity into account you're going to be pretty far off. :)

I understand the odd number of disks thing - this disk group actually has 161 disks but 1 of them is ungrouped for this reason (best practice of having disks in bunches of 8 etc etc).

Apologies for not closing this thread yet everyone. My numbers so far are saying that if we convert all the Vraid1 vdisks to Vraid5 we'll save almost 12TB in this DG alone ... for obvious reasons I don't want to say that to the customer unless it's correct (that's a lot of disk space for this particular customer).