How to view RSS on EVA! + Expansion

[Víctor Cespón]

@sam bell

"disk failure protection" always reserves 2 x (biggest disk on disk group), to have enough space to rebuild in the worst case scenario.
Does not matter if you have vdisks in RAID 1 or not, it will reserve this space.

@Useless1
Once a disk fails, the rebuild process regenerates the missing data and puts it on the remaining disks on the RSS.
Once the rebuild process completes, we're back to the fully redundant state, so we can lose another disk.

RAID failure happens only if a disk fails before an ongoing rebuild completes.

After rebuild completes, RSS reorganization or leveling will be launched.

It's easy to see with RAID 1:

When a disk fails, we have a set of data that's only on one disk. We must have two copies of this data, so we copy the data to the other disks on the RSS. Each data segment on two different disks.

Until this process has been completed, we cannot lose another disk, if we lose the disk containing the only copy of the data, before we have 2 other copies on two other disks, there's no way to recover the data.

As you can imagine, losing two disks that are a mirror in the few hours that a rebuild takes, is very unlikely.

Disk failure protection only reserves free space to perform rebuilds, does not enhance the rebuild process.

[UselessUser]

Thanks for this, I am reaching clarity!

So technically with protection level set to single on a maxed out EVA so there is no free space to use first before the reserved protection space...

With single protection we can technically lose any 2 physical disks before losing data, as long as a rebuild completes before losing the second.

The issue of course is, if this happens, the array will not have any spare space to rebuild the redundancy and will be operating in degraded mode - calculating data on the fly?

[sam bell]

Useless,

could you assign some points in this Thread please as I think it evolved to a really good source for understanding RSS/redundancy on the EVA. People should see this at first glance that this thread has good (magical :) answers.

Thanks

[Víctor Cespón]

@Useless1

OK, let's say I have a 32 disk disk group filled with vdisks / snapshots / snapclones, to more than 99% capacity.

1) This is not recommened as the leveling process needs some space to work. Minimum is 5 GB free.

2) If while performing a snapshot the EVA runs out of space on the disk group, the snapchot will go to "snapshot overcommit" state and all snapshots will be deleted.

3) If the EVA is replicating with another one and the disk group is almost full, in the event of a link failure or a DR Group being suspended the log disk will run out of space and will be invalidated, this will then force a full copy of the DR group.

So, I have 32 disks and one fails. RAID 1 data is stored on pairs of disks. Disk 31 has no pair, so we must get all the RAID 1 data out of it and copied to a pair of disks.

While doing this, another disk fails. If it's disk 31, then we have lost the only copy of the data. If it was on the same RSS as 31 but it's not disk 31, then we lose RAID 5 but not RAID 1.

If it's on another RSS, now we have to perform rebuild on two RSS, also, this will trigger a change on the RSS as we cannot have two RSS with an odd number of members.
As we have 2 disks that have no partner, one will be moved to the other RSS and they will form a pair to store RAID 1 data.

If there's no free space anywhere to perform rebuild, the RAIDs will work on degraded mode. RAID 1 with only one copy of the data, RAID 5 generating the missing data from the 3 data and 1 parity left.

So you see, it's complex... is not "I can lose 2 disks"... it all depends on what disks and when each failure happens.