What capacity with VA7100

[sid_22]

Hi for all


We have VA7100 in AutoRAID mode with 5 x 18 Go disks (with 1 HotSpare disk, plese see attached file). We want to add either : 10 x 36 Go disks or 5 x 73 Go disks and we want to know what will be the capacity in the two cases

Regards

[TTr · Accepted solution]

If you look at the "CAPACITY USAGE" in your attachment, you see the following

Total Physical Size: 83.446 GB
Allocated to Regular LUNs: 24.195 GB
Used as Active Hot Spare: 16.687 GB
Used for Redundancy: 36.809 GB
Unallocated (Available for LUNs): 5.753 GB

As you can see, the problem with 5 disks is that there is too much overhead. You use one disk for hot spare and then a lot of space is used for autoraid redundancy.

For 5 x 73 disks you can quadruple these numbers. Useful LUN space will be about 120GB.

With 10 drives the overhead is a little less. I can not tell exactly what all the numbers will be for 10 x 36 drives but you should expect anywhere from 180GB to 210GB useful space for LUNs. Maybe something like this.

Total Physical Size: 333 GB (exactly)
Used as Active Hot Spare: 33.3GB (exactly)
Available raw disks space: 300GB
Used for Redundancy: ~100GB
Available to Regular LUNs: ~200GB

The performance will be better with 10x36GB disks because you will have 10 disks doing i/o instead of 5.

With 5 disks you should consider using RAID1+0 instead of AUTORAID. You get more useful space for LUNs.

One last thing. You have assigned 0 points to 30 responses. You should assign pouints. That way you get more responses very quickly.

[sid_22]

Hi

Thanks for your response. Sorry, I will pay attention for assigning points.
If I understand in theory 5 x 73 disks and 10 x 36 disks have the same amount space (about 360 Go) , but 10 x 36 disks provide between 30 and 80% more space in AutoRaid ? Can you explain please

Regards

[TTr]

For all calculations I used the true size of a disk. In your array right now, the 18GB disk is really 16.7GB. The 36GB disk is 33.3GB and the 73GB disk is 67GB.

Yes, 5x73 and 10x36 have the same amount of storage but immediately you use one disk for hot spare in each case, so in the first case you lose 1x73GB (that's 20%), in the second case you lose 1x36GB (that's 10%). Now you have 4x73GB=(4x67)=268GB or 9x36=(9x33.3)=300GB available for RAID. AutoRAID uses its own RAID mechanism for redundancy which is a combination of RAID5 and RAID1. But with few disks (4), the autoraid overhead is high as you can see in your existing setup right now with the 5x18GB disks. With 4 disks a lot of space is taken up in the mirroring (RAID1) and parity (RAID5) schemes of autoraid. With 9 disks, the overhead is much less. There are NO known formulas that we can use for calculating the autoraid overhead.

With 5x18GB disks you have this setup right now and you can see the results. The exact same thing will happen with 5x73GB disks.

I have several VA disk arrays and in one VA7100 with 15x36GB disks setup as autoraid, the numbers are

Total Physical Size: 500GB (15x33.3)
Used as Active Hot Spare: 33.3GB (1 drive)
Available raw disks space: 466.7GB (9 drives)
Used for Redundancy: 110GB
Available to Regular LUNs: 356GB

So based on the 5disks that you have and the 15 disks that I have, I concluded that if you get 10x36GB disks you will get about 200GB for LUNs as I mentioned in my previous reply.

[sid_22]

Hi

I understand your reasoning but please donâ t forget that in our VA we have already 5 x 18 Go and want to ADD either 10 x 36 Go or 5 x 73 Go
So the final VA configuration will be with mixed disks capacities. Actually it will be one of the following:

A. 5 x 18 Go + 10 x 36 Go
B. 5 x 18 Go + 5 x 73 Go

In the VA we have already a 18 Go HotSpare disk. So in the worst case I expect to ADD the following capacities :


A. + 10/2 x 33.3 Go
B. + 5/2 x 67

Is that correct ?

Thanks

[TTr]

You are using mirroring in your calculations. It is good for a worst case scenario but the existing 18GB hot spare can not cover for the 33 and 72GB disks. So it would be more like
9/2 x 33.3 or 4/2 x 72GB

[sid_22]

thanks