eva stripping

[Kenan Erdey]

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.

[Steven Clementi]

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

[Uwe Zessin · Accepted solution]

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]

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.