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redundant storage sets (RSS) in EVA

 
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Suhag
Occasional Advisor

redundant storage sets (RSS) in EVA

Hi All,

 

I want to know about redundant storage sets (RSS) in EVA. should we consider this while designing the Disk group, RAID level and LUN size ??

 

Thanks,

 

Suhag.

7 REPLIES 7
Johan Guldmyr
Honored Contributor

Re: redundant storage sets (RSS) in EVA

Hello!

Somewhat, you don't have much control over it though.

For example it's best if the amount of disks in a DG is divisible by 8 - as then optimally each group 8 disks will be in one RSS.
Suhag
Occasional Advisor

Re: redundant storage sets (RSS) in EVA

Thanks for your response, So it is thumb rule that for every eight disk, there is one RSS. I have configured one disk group (with double protection level) with 17 no of 1 TB hdd, so how many RSS will be there ? Thanks, Suhag.
Johan Guldmyr
Honored Contributor

Re: redundant storage sets (RSS) in EVA

1 DG with 17 disks will be two RSS: one with 8 and one with 9. That's if you put them in at the same time. If you now add one disk it will probably add one to the RSS with 9 members. One more: 8+11. One more: 8+6+6 (so split the 2nd one).

 

If you have an uneven amount of disks in a DG beware that you will not use all disks in vraid1, because the 17th disk has no buddy to mirror with.

 

I think you can see the RSS ids on the physical disks in the controller configuration dump (inside the .xml file).

Suhag
Occasional Advisor

Re: redundant storage sets (RSS) in EVA

fair enough Johan,

 

one last query before i go for "Accept As Solution" ,

 

I have configured dual disk protection level for the said disk group. I have combination of Raid5 and Raid 1 vdisk in the same Disk group. 

 

Now when two no of disks will fail from the same RSS simulteniously, will all the data survived ?  Is disk protection level releted to no of disk fail in disk group ?

 

Thanks,

Suhag.

 

 

Johan Guldmyr
Honored Contributor
Solution

Re: redundant storage sets (RSS) in EVA

Hi,

no it is not.
Disk Protection level reserves the equivalent of 4 of your largest disk in the DG. So that would be 4TB for you. Usually people have this as single.

In your setup:
You can lose one disk in each RSS until the vraid 5 fails. If you lose two disks in the same RSS, vraid5 data does not survive.

You can lose half of the 16 disks that are used for vraid1 and data will survive, but you need to be 'lucky'.

There are lots of other threads on this forum about RSS, I would suggest that you go look them up :)
noori-butt
Frequent Visitor

Re: redundant storage sets (RSS) in EVA

what will be the effect on Raid 1 or Raid 5 level when one or two disks are removed from RSS?

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Greybeard
Esteemed Contributor

Re: redundant storage sets (RSS) in EVA

A brief explanation of EVA data protection within RSSs and group protection levels in a disk group.

With one disk group which has 24 300GB SAS disks which shows a capacity of 5583.4GB and another disk group which has 8 1TB FATA disks which has a capacity of 3723.78...

How does this work out when technically the FATA disk group has more space but less is available??"

First question... what IS your Disk protection level for both of these groups?

Well with 'protection level' double the EVA reserves 4x the size of the largest disk in the disk group for data rebuilds and distributes the capacity across all physical disk drives in the group.
Think of it as the capacity of 4 disk drives is 'rotated' by 90 degrees and 'striped' across all disk drives.

Why the FATA group has less free capacity? A larger reservation, because a single drive is sooo much larger:
4*300GB = 1,200GB
4*1000GB = 4,000GB

((24-4)*300 HWGB)/1.024/1.024/1.024 ~= 5587 SWGB

((8-4)*1000 HWGB)/1.024/1.024/1.024 ~= 3725 SWGB

HWGB is 'HardWare GigaBytes' (number of blocks on the disk drive * 512 Byte/block divided by 1,000,000,000)

SWGB is 'Software GigaBytes' - Command View EVA divides by 1,073,741,824 = 1073741824
(1024 = 2^10)

When a disk group is created the EVA will divide the total number of disks into groups of 8 'failure domains' called RSS (redundant storage set). In the event of a disk failure a reconstruct is done completely within that RSS first. Only after it has finished is the data is 'leveled' across all disks in the group.

Then you have the Vraid

Some traditional RAID arrays can do between 2/3 and 13/14, depending on the number of disk drives (3 to 14 in this case).

The EVA 'virtualises' 4D+1P RAID-5 and distributes it onto different disk drives within the disk group. This even works in a disk group with 240 disk drives. In that case the data rebuild for a failed disk drive is MUCH faster than rebuilding a 239D+1P array.

Another reason for smaller RAID-5 chunks is the (potential) ability to withstand multiple concurrent disk drive failures within a disk group. It is not a true RAID-6, but the disk group is divided into multiple 'failure domains' - the RSS (Redundant Storage Set) - each of which contains isolated RAID protection.

Internally the data for a vRAID 5 logical drive is segmented and stored as 4 data segments with one parity segment. And these segments are written on groups of 8 disks (usually 8 disks, which from a RSS)

The issue of "disk failure protection" = single requiring 2 drives is because if a disk fails and there are logical drives in vRAID 1, you need to move the data that was on the failed disk and its partner to the remaining disks, so there are two copies of each data.
To be able to do this even if the disk is all VRAID 1, the controllers reserve twice the size of the biggest disk on the disk group.

basically what happens is...

D1 paired D5 (data)
D2 paired D6 (data)
D3 paired D7 (data)
D4 paired D8 (reserved for single protection)

If you have a vRAID1 disk allocated.. and D1 fails... it moves/copies the data from D1/D5 to D4/D8.

To put it another way, single/double protection keeps you from having to figure out how much space to leave in your disk group to survive a disk failure - Single leaves enough space to survive a single failure, double allows you to survive two failures at the same time, assuming, of course, your VRAID1/5 setting has basically copied your data to other disks to allow the EVA to redeploy the data once a failure happens.

So the EVA uses RSS, which splits the disk group (if more than n disks) into separate RSS's.

Each RSS can hold between 6 and 11 disks and if a disk group has 11 disks and a 12th is added, the EVA automatically creates another RSS and reallocates disks according to an algorithm (which prefers all groups to contain 8 disks).

Using VRAID-5 a minimum of 8 shelves is recommended... this is because along with the recommendation of multiples of 8 disks, this could survive the failure of a shelf. E.g. you have 16 disks in 8 shelves with 2 disks in each shelf, the EVA would create 2 RSS groups, each with 8 disks, ideally with each RSS having 1 disk from each of the 8 shelves. Now if 1 shelf failed each RSS would have 7 disks, and as RAID 5 allows one disk to fail because it can calculate the missing information from either the parity bit, or recalculating the parity bit from the actual data...

The VRAID-5 implements a 4D+1P redundancy, but it does not use a fixed set of 5 disks. A 4D+1P stripe is always stored on 5 _different_ disks (that's why an RSS uses 6 disk drives minimum - if one fails, full redundancy can still be restored) -- think of VRAID-5 data as 'shuffled' across all disk drives in an RSS.

Therefore eight shelves (or more) with disks added vertically is the optimal configuration, as the controllers will create each RSS so that no two members exist on the same shelf, also each RSS member has a number and the algorithm tries to ensure that no two disks with the same number, from different RSSs, are created on the same shelf.

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