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How to view RSS on EVA! + Expansion

 
UselessUser
Frequent Advisor

How to view RSS on EVA! + Expansion

Hi,

As a follow up to my previous post. I currently have a EVA 4400 with 4 shelves and 24 FSAS disks.

I have the protection level set to double, simply because the last two times I have needed to replace a disk it has taken over a week both times to replace in line with HP Best Practices config guide.

The person who set it up used VRAID 5 to create LUN's, which means I have the ability to lose a single disk at a time in each RSS before data loss. (But if reconstruction completes after the first failed disk I can lose one more from each RSS due to double "protection")

I believe that because I only have 4 shelves I do not have shelf redundancy at the moment, because the parity and data chunks of the VRAID 5 LUN's could reside on disks in the same shelf?

Now if thats the case, I believe I have 2 options to give me shelf redundancy, either buy 4 more shelves and re-arrange my existing disks across them, or convert all my VRAID 5 arrays into VRAID 1 arrays, because I am led to believe that VRAID 1 would split each mirror pair ensuring that they do not reside on the same shelf? Can I verify this in anyway on my EVA using some commandline?

If I were to buy the shelves, how would I add them and redistribute the disks and keep it online at the same time? And once this is done can I view the RSS layout to verify it has done the right thing?
23 REPLIES 23
IBaltay
Honored Contributor

Re: How to view RSS on EVA! + Expansion

Hi,

1 RSS member=1 physical disk of the same RSS in each disk shelf is giving you full verticality both for RAID5 and RAID1...

2 the current (non vertical) and future RSS layout (vertical) can be checked via SSSU CLI

3. the redistribution can be done online via the ungroup/reshuffle/group of the "RSS horizontal" to become new "RSS vertical" in the new positions in the new disk enclosures (this is online but time consuming)

4. or it can be done offline after the EVA shutdown at once. (offline but quick)

the pain is one part of the reality
UselessUser
Frequent Advisor

Re: How to view RSS on EVA! + Expansion

Hi,

I am sorry I do not fully understand the answer 1??

Does this mean I am correct in what I am saying about shelf redundancy at this moment in time???
V├нctor Cesp├│n
Honored Contributor

Re: How to view RSS on EVA! + Expansion

Several misconceptions here:

1) You do not need "disk failure protection" = double on an enclosure with 24 disks. It's only needed on enclosures with more than 100 disks and where an spare disk can take several days to get there.
"disk failure protection" = double does not mean you can lose two disks from the same RSS. It means there are space reserved to be sure to be able to perform two rebuilds.

2) Currently you cannot see the RSS state in Command View. It can be deduced from the SSSU output, but has to be done manually.

3) Even if you add another 4 enclosures to have 8, the RSSs will not be automatically modified so there's one disk on each enclosure.

4) Even if you move disks around to reach that state, it can change at any moment, after you add or remove a disk.
IBaltay
Honored Contributor

Re: How to view RSS on EVA! + Expansion

Hi,
1 RSS member=1 physical disk of the same RSS in each disk shelf is giving you full verticality both for RAID5 and RAID1...

this was meant in the sense that if you have only 1 RSS member in the disk enclosure, then there is no double disk failure of the whole DG in case of the possibility (even very rare)
of the whole disk enclosure failure.

the pain is one part of the reality
sam bell
Regular Advisor

Re: How to view RSS on EVA! + Expansion

The RSS layout can bee seen in Command View by opening it the following way:

https://server:2372/nsafieldservice.htm

Just select the appropriate EVA and choose "Disk Groups and Redundant Storage Sets (RSS)"

@vcespon: Though with double disk drive failure protection he is not protected for loosing two disks of one RSS at the same time it however means that he can loose a second drive from one RSS once the first failed drive has been reconstructed.
V├нctor Cesp├│n
Honored Contributor

Re: How to view RSS on EVA! + Expansion

@sam bell
Please DO NOT post HP restricted information on a public forum.

That page has a big red box saying:

The features offered in this menu are intended for use by authorized Hewlett-Packard service engineers only. If not properly used, some features can cause data loss or corruption. Do not use these features unless you are authorized to do so.

Several people in this forum have access to HP internal advisories, tools and documentation, but we refrain from posting things that are not public.
UselessUser
Frequent Advisor

Re: How to view RSS on EVA! + Expansion

I do not understand this still...

"disk failure protection" = double does not mean you can lose two disks from the same RSS. It means there are space reserved to be sure to be able to perform two rebuilds.

My thought for this was from this:

"As an example, a Protection Level of 1 provides continued operation in the event of two disk failures, assuming the restore from the first failure completes before the second disk fails."

However thinking about this is the reason it cannot survive 2 disk failures from the same RSS is because if I lose a disk in the RSS, it will then rebuild the data from this disk over to the remaing disks evenly within the entire disk group. However some disks will be in the same RSS set as the failed disk, which means with VRAID 5 I get into a situation where one of the remaining disks in the RSS has both a parity and data chunk for each ?kb stripe, and therefore losing one more breaks my RAID5? (Which leads to the obvious questions can I lose more than 1 disk in an RSS if it is using VRAID1 LUN as long as it is not 2 of the same pair at the same time.. and reconstruction from the first failure has finished?)

I also do not understand the recommendation of double protection unless you have over 100 disks AND it takes a long time for a replacement to arrive. I got my idea for double from this:

"Conversely, the statistical availability of disks and the typical service time to replace a failed disk (MTTR2) would indicate that a Protection Level of two would be unnecessary in Disk Groups of fewer then 168 disks in all but the most conservative installations. A mitigating condition would be if the service time (MTTR) might exceed seven days, then a Protection Level of 2 might be considered."

I just do not think I understand exactly where this protection level comes into play. And how crucial it is, as it sits outside of the VRAID setups

I would have assumed if I gave the EVA the best opportunity to create redundancy (ie blocks of 8 disks spread evenly over 8 shelves) it would do its hardest to align itself for that purpose (shelf redundancy) but obviously I am wrong.

I got the whole idea of using VRAID 1 because I have less than 8 shelves from this taken from the best practices guide:

"The highest level of redundancy is achieved when eight or more disk shelves are used. In this way, the array minimizes the conditions in which the RSS has two disks in the same disk shelf."

"With Vraid1, the EVA firmware attempts to place the individual members of a mirror pair on different shelves. Because of this, the guidelines are much simpler, and the suggested number of shelves can be less than eight"

I am happy to bow down to other people's advanced knowledge, hence even asking the question.
IBaltay
Honored Contributor

Re: How to view RSS on EVA! + Expansion

Hi,

i ll try to give u the example for RAID1:

1. Single disk failure (rebuild)
if one disk carryng the RAID1 stripes of the RSS fails, then it is only a 1 disk failure of the RAID1 mirror set and all its mirror stripes will replicate to other RSS members as well as it will be mirrored to the adjacent RSS members in the new places to create the complete mirrored set.

2. Double disk failure (rebuild)
if one disk of the RSS fails and carries the RAID1 stripes, and at the same time the second failed disk does not hold any of the mirror stripes of the first failed disks, then the rebuild will start as described in the variant 1

3. Double disk failure (data loss)
if one disk of the RSS fails and carries the RAID1 stripes, and at the same time the second disk holding the "secondary" mirror stripes fails, then it is a double disk failure of the whole disk group
the pain is one part of the reality
sam bell
Regular Advisor

Re: How to view RSS on EVA! + Expansion

@vcespon: Sorry, didn't know that it's not permitted to publish such information on the forum and won't do it again. I however think it's less than optimal that we don't have any option to view the RSS information in Command View. I mean, even though the EVA takes care about the RSS design and usually you don't have to worry about it, in general the purpose of RSS is important to understand and except the best practices whitepaper the whole thing is kept out of the loop.

@Useless1

> However thinking about this is the reason
> it cannot survive 2 disk failures from the
> same RSS is because if I lose a disk in
> the RSS, it will then rebuild the data
> from this disk over to the remaing disks
> evenly within the entire disk group.
> However some disks will be in the same RSS
> set as the failed disk, which means with
> VRAID 5 I get into a situation where one
> of the remaining disks in the RSS has both
> a parity and data chunk for each ?kb
> stripe, and therefore losing one more
> breaks my RAID5? (Which leads to the
> obvious questions can I lose more than 1
> disk in an RSS if it is using VRAID1 LUN
> as long as it is not 2 of the same pair at
> the same time.. and reconstruction from
> the first failure has finished?)

I'm currently thinking about this too and I think that in a following configuration the theortical behaviour is like the following:

Given on an EVA4400:

* 24 disks
* 3 RSS with 8 disks each
* LUNs: 3x Vraid 5, 2x Vraid1, 2x Vraid6
* Protection Level 1

If I understand correctly, then:

- I can loose one disk in each RSS without affecting any of my Vdisks. I can loose a second disk in one of those RSS if the disk that failed first has been reconstructed to the free/protection level reserved space.

- If two disks in one RSS fail simultaneously I'll definitively loose *all* my Vraid5 Disks (since the Vdisks are striped over the whole disk group) and possibly also all of my Vraid 1 LUNs (only if the failed disks were married to one pair).

- The Vraid 6 LUNs can withstand a simultaneous fail of two disks in each RSS, which means a total of six drives (two per RSS).

- The Vraid 1 LUNs can withstand a simultaneous fail of four disks in each RSS as long as no married pair is affected, which means a total of 12 drives (very unlikely but theoretically possible).

Not exactly sure though.