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More about EVA RSS and protection level

jpmartin
Regular Advisor

More about EVA RSS and protection level

Hi,

i would have some precisions about EVA RSS and protection level as described in previous note :

http://forums1.itrc.hp.com/service/forums/questionanswer.do?threadId=1111467

If i take an example of an 6100 model 2C6D with 84 drives, installed on 14 columns of 6 drives, with one Disk Group, i may get 14 RSS's, right ?

If he DG has a protection level 2, it can loose 4 drives without problem.

If a shelve is broken, each RSS loose a drive, but still alive.

The question : in this case, the DG is loosing more than 4 drives , what aout the state of the DG?

- alive because each RSS still alive
- dead because more than 4 drives failed

Regards

Jean-Pierre
9 REPLIES
Rob Leadbeater
Honored Contributor

Re: More about EVA RSS and protection level

Hi,

The EVA will normally try to get 8 drives in an RSS, so with 84 drives you would probably get 10 RSSs (I think).

If the disk group has double sparing, then 4 times the size of the largest disk in the group is reserved. This doesn't necessarily mean that you can lose 4 drives without a problem... For example consider the situation where there is a Vraid-1 disk in that group. In this situation data is mirrored in pairs within the RSS. If both drives of this pair were to fail then you would have data loss.

If you have any Vraid-5 drives in your disk group, then you need to have at least 8 shelves to be able to sustain the loss of a shelf - and then the disks within each RSS must be distributed so that each is on a separate shelf.

Hope this helps,

Regards,

Rob
Jonathan Harris_3
Trusted Contributor

Re: More about EVA RSS and protection level

You won't get 14 RSS groups, because RSS groups have a target size of 8 disks (with a minimum of 6 and a maximum of 11). It will fill up RSS groups in multiples of 8 first and then calculate the spare.

With 84 disks you would get:
8+8+8+8+8+8+8+8+8+6+6

You could in theory, lose 11 disks, provided they were all in different RSS groups before affecting a VRAID5 disk, or you could lose up to 42 disks before affecting a VRAID1 disk (but by then the metadata would have failed).
jpmartin
Regular Advisor

Re: More about EVA RSS and protection level

OK,

thank you for the inputs,

so, i've read before (i don't remember where) that firmware XCS is able to align RSS's in groups of 6 drives, if it detects 6 shelves, may be i wrong ?

If it is not possible, the configuration will have 8 shelves with 80 disks to obtain 8 RSS's as you described before.

The precedent calculation was 80 disks, and was fixed up to 84 to have 14 columns.

The configuration will be made of Vraid5 VD's only.



jpmartin
Regular Advisor

Re: More about EVA RSS and protection level

Sorry for the mistake,

80 disks in 8 shelves --> 10 RSS's of 8 disks
Rob Leadbeater
Honored Contributor

Re: More about EVA RSS and protection level

Hi,

The best practice guide here:

http://h71028.www7.hp.com/ERC/downloads/4AA0-2787ENW.pdf?jumpid=reg_R1002_USEN

still mentions the target of 8 drives per RSS. I've not seen anything that suggests that you can get 6 drives in an RSS with 6 shelves - although I could be wrong.

Cheers,

Rob
jpmartin
Regular Advisor

Re: More about EVA RSS and protection level

OK Rob,

so, i have found the document which refers to initialize RSS's with 6 members in the case of 6 shelves.

It is dated of begin 2005 and was a presentation done by HP to partners before the annoucement of EVA4000/6000/8000.

This feature was showed as an option, but may be never coming, i don't know.

Thank you for help.

Jean-Pierre

Uwe Zessin
Honored Contributor

Re: More about EVA RSS and protection level

> done by HP to partners before the annoucement of EVA4000/6000/8000.

Ah yes, I think I know what presentation you are talking about ;-)
In that case it was an NDA session talking about potential new features.

The problem is:
- not all those great things get implemented
- some other things are implemented, but not documented
.
jpmartin
Regular Advisor

Re: More about EVA RSS and protection level

That's right,

so, i follow the actual best practises with 8 shelves to obtain RSS's with 8 members.

Regards

Jean-Pierre
jpmartin
Regular Advisor

Re: More about EVA RSS and protection level

N/A