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eva stripping

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Kenan Erdey
Honored Contributor

eva stripping

hi;

i have a a question about eva. imagine we have aa raid 1 disk. when data is being stripped, are only 4 disk of RSS being used (i mean the the other 4 is used for mirroring), or are all disks in RSS used for stripping ?

for example a raid 1 vdisk with 8 disk.

DISK 1 2 3 4 5 6 7 8 DISK 1 2 3 4 5 6 7 8
data a a b b c c d d or data a b c d e f g h
data e e f f g g h h data b a d c f e h g

thanks.
Computers have lots of memory but no imagination
3 REPLIES
Steven Clementi
Honored Contributor

Re: eva stripping

I can not say for sure since I did not study the internal workings of the EVA Controller, but I would think that ALL of the RSS members would get a chunk of the data, and then a chunk of the mirrored data where that chunk of the mirrored data is on a different disk then it's "parent" data.


Steven
Steven Clementi
HP Master ASE, Storage and Clustering
MCSE (NT 4.0, W2K, W2K3)
VCP (ESX2, Vi3, vSphere4, vSphere5)
RHCE
NPP3 (Nutanix Platform Professional)
Uwe Zessin
Honored Contributor
Solution

Re: eva stripping

I have never seen any 'official' documentation on how the data is layed out on the disk drives - it can even change over time. It's been a year or two ago I have attended a session on the EVA where it was explained a bit more, but still not at that detail.

The system always picks up 4 disk drives from an RSS to make up a full stripe. In case of VRAID-1, the data is stored on two different disks, but a full striped mirror might not cover 8 disks, because an RSS can be as small as 6 disks. In case of VRAID-5, 5 disks are choosen to store data+parity (4D+1P).

Of course, the controller firmware does not simply stripe over the disk drives. It first divides each disk into so-called PSEGs (physical segments) of 2 MegaBytes size - if this is still the case on the EVA XL, I don't know.

A virtual disk is divided into many RSTOREs (redundant stores) of 8 MegaByte size. An RSTORE is logically made of 4 x 2 Megabyte PSEGs.

For VRAID-0, this means 4 PSEGs on 4 different disks. For VRAID-1, this means 8 PSEGs on 6-8 different disks. For VRAID-5, 5 PSEGs on 5 different disks.

Want to use the other disks in the same RSS, which can be as large as 12 disk drives? - The system allocates them to other RSTOREs.
.
m saravanan
Valued Contributor

Re: eva stripping

Kenan..

While you are creating Vraid1 the disks are grouped as a pair. If any unpaired disk is remained, that won't be used for Vraid1 LUNs. For Ex, If the disk group is containing 9 HDDs, the first 8 (even numbers) HDDs will be grouped as a paired. The unpaired 9th HDD will not be used for Vraid1.

So the data will be stroed as like,
a a b b c c d d

If you are using Vraid5, the stripe set will be always 4.

Regds,
Saravanan.