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EVA 4400 disk space calculation

Posted on 2009-07-02
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We have just added a new disk group of 10 X 450Gb drives to our EVA, set with a Failure protection of 1. Occupancy is 0 at this point (no Vdisks created).
When I select the disk group I am seing a total capacity of only 3350Gb. I understand the need for rebuild space etc but this seems excessive.

Does anyone have a good explanation of why so little space is available for use?
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Question by:agradmin
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by:garycase
garycase earned 100 total points
ID: 24770436
I don't have a StorageWorks EVA 4400, but I think this is simply a combination of two things:

(1)  You have a spare drive assigned ... so only 9 drives are actually used for the array => thus the capacity is (9-1) * (450GB) = 8 * 450 = 3600GB

(2)  The normal difference between "computer-ese" and "disk-drive-maker-ese" in the way storage is measured [i.e. 1GB in "computerese" = 1024 x 1024 x 1024;  1GB in "disk-drive-maker-ese" = 1,000,000,000].
Note that 3600GB in "disk-drive-maker-ese" is 3352.76GB in "computerese" ... just about what you're seeing.
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andyalder earned 200 total points
ID: 24771599
It's not actually down to RAID parity, disk failure protection (none, single or double) is the level of distributed sparing, you have this set to one, you would expect to lose one disk's capacity but it is one spare per 8 disks (EVAs do most things by 8s) so with 10 disks you have 2 distributed spares hence you've lost 2 disks worth of data.

You can tell this because if you use just 8 disks with disk failure level set to none you'll have the exact same capacity as you have with 10 disks and disk failure protection set to single. (I don't have an EVA either but I have a simulator).

Then if you use vRAID 5 you'll again lose 2 more disk's capacity because vRAID 5 works in 8s as well, it's a form of RAID 50 with 8 disks per RAID5 group. Note though that this won't be 2 * 450GB but 2 * 335GB because in effect you have already reduced the size of the disks because of the distributed spares.
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by:raj27962
raj27962 earned 200 total points
ID: 24786624
i have multiple eva's and andyalder is right - failure protection of 1 is actuall 2 disks - failure protection of 2 is 4 disks, though u dont need that till you hit your 4th shelf - in effect your losing 2 x 450 disks for the failure protection and then you get an actual presentation of space ( i.e. calculating space using 1024 not 1000 )

Just to really rub it in - you dont have 3350Gb - you have 2847.5 GB of space according to HP's recommendations - since you need to leave around 15% of the disk space for the eva to do its magic
( moving data across the disks to optimise access ) - if you start eating into the 15% it needs then you'll degrade performance which isnt something you want to do on an expensive system like an eva

On the bright side you wont to lose more disks to failure protection for the next 3 shelves. Also remember not to create more disk groups - each one will take disks for failure protection - in an eva you just want the one disk group for the 450 gig fc drives - create extra disk groups if you add in the 1 terra satas for nearline backup etc and so on
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by:andyalder
ID: 24787139
Hmm, you're right raj27962, it's only 2 disks worth for single sparing right up to 48 disks in the group, I thought it was like the one per 8 of vRAID5.
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by:agradmin
ID: 31599303
Thanks for all your help - at least now I know I'm not going crazy!
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