Space savings of Vraid5 vs Vraid1

[Chris Rasmussen [FAU]]

I've got a 1.2TB Vraid1 vdisk created and presented - it's working fine.

If Vraid1's parity says that the entire size of the vdisk is mirrored, that means it's actually using 2.4TB of space on the array.

Now, if I was to change the vdisk from Vraid1 to Vraid5 and, knowing that the space overhead on Vraid5 is 25%, does that mean that the space used by this disk in Vraid5 configuration would be (in TB):

1.2+(1.2*0.25)

This is ~1.5TB, ignoring the 1000Gb vs 1024Gb topic for now. That's a 900GB saving if I was to change the vdisk from Vraid1 to Vraid5.

Is this correct? I'm not concerned with performance right now because EVAPerf tells me we're well under the available I/O for this particular disk group.

[TTr]

> knowing that the space overhead on Vraid5 is 25%...
> This is ~1.5TB...

It is actually 20%. Vraid5 is using a 4D+1P model and it is based on blocks and not spindles. That is the 4D+1P are 4 data and 1 parity blocks that are spread on sets of 5 spindles at the time (but not the same 5 spindles but as many as the raid groups has in it). i.e the first set of 4D+1P blocks is using a set of 5 spindles, the second 4D+1P blocks is using a *different* set of 5 spindles and so on until all spindles in the groups are equially utilized and used up.

So it is really (1P)/(4D+1P) = 20% of your overall space.

I think the 25% is obtained based on the usable disk space, i.e. (1P) / 4D = 25%.


So, you should expect around 1.6TB of usable space. Now adding hot spare space complicates things a little bit because the hot spare space is still virtual and distributed to all disks but it is the equivalent to the space of 1 spindle.

[Chris Rasmussen [FAU]]

Sorry TTr, not sure what I was thinking there - I meant 20% throughout that post. My mistake!

So, the basic calculation I used makes the total used space for a 1.2TB vdisk in Vraid5 configuration ~1.44TB ... how did you arrive at 1.6TB?

Sorry, brain's not working today. :(

[TTr]

I miscalculated as well. I thought you had a 2TB volume.

If you assume 2.4TB of spindle space,
it is (2.4TB) * (0.8) = 1.92TB.

If you use sparing, you have to subtract the space of one spindle.

Either way it will much more than the vraid1 space.

[Chris Rasmussen [FAU]]

Hehe that's cool.

I have a 1.2TB vdisk right now. 2.4TB is what I thought would be used in *spindle space* for a 1.2TB volume. Is that right?

Are you saying that Vraid5 will use more spindle space than Vraid1??

[gstonian]

I'm not sure of your set up but by my calculations, if you had a small disk group say 12 x 300GB Disks with 90% occupancy alarm, you could create either

a) 1,257GB Lun at Raid1
b) 2,011GB Lun at Raid5
c) 2,514GB Lun at Raid0

I realise you don't have this setup but the available space should still apply and hopefuly gives you a guide to what can be created with the available space.

Regards

[TTr]

The spindle space is the amount of actual disk space on the physical disks that vraid1 is currently using. How many disks and what size are they? Also if you have commandview you can look at the detailed information for the current vraid1 set up.

[Chris Rasmussen [FAU]]

Ahh yes see this is where the problems begin. The disk group that contains the 1.2TB Vraid1 vdisk is one of 2 in this array and has 160 disks.

They're all 15k FC but are not all the same size - a mix of anything from 72GB to 300GB as far as I know (EVA 8k which has been added to over the years).

For this reason I can't use StorageWorks SAN Sizer to do any calculations or estimates (gstonian I assume that's what you used to get those numbers?)

TTR, yes of course we have CV EVA (9.1) but which detailed info do you mean?

Thanks for all your answers so far, you are helping. :) Just not quite there yet ...

[Uwe Zessin]

Outch, Outch...

Of course you can't use a simple formula when mixing disk drive sizes in a disk group. It depends on the exact (logical) position of the disks in a group how much space can be used on any particular disk drive. And this can easily change when disk drives are added or removed from the group.

[Chris Rasmussen [FAU]]

Some basic calculations I did, based on 20% overhead for Vraid5 and 100% 'overhead' for Vraid1 were:

500GB Vraid1 = 1GB used DG space
500GB Vraid5 = 600GB used DG space
100GB Vraid1 = 200GB used DG space
100GB Vraid5 = 120GB used DG space

These numbers, if correct, ignore sparing but I'm hoping they're reasonably close. It looks like I don't have those numbers correct though?

The whole point of all this is so that I can make recommendations to the customer about how much space we could save if we changed some of the low I/O Vraid1 vdisks to Vraid5.

Hopefully that additional info helps!