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recoverability and multiple storage pools in a volume

jdub's picture
Submitted by jdub on Thu, 06/16/2011 - 2:47pm
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General Xsan Talk

Hello,

I'm going to be adding an other storage pool to an xsan and am trying to figure out if I should expand the existing volume with a new storage pool, or to create a new volume from the pool.

The main question I'm trying to resolve involves recoverability. Backups aside, I've been asked to create a setup where some set of files are on both pools so that the data is still immediately available in case one of the LUNs has a failure.

Some things I've read indicate that if any LUN in any of the pools that are part of a volume fails, the entire volume is destroyed (presumably including the data on the unaffected storage pool.

However, the training docs say: "Each storage pool is assigned an affinity tag according to the pool's performance and recoverability", which seems to indicate that recoverability is specific to the underlying pool, not the entire volume.

I would rather expand the existing volume rather than make a new one so that users don't need to keep track of 2 separate volumes. But I don't want the new (larger) pools to be vulnerable to a failure in the older pool.

Does anyone have experience with losing a LUN from a volume with multiple pools? Does it render the entire volume unusable? Is there any other reason why I may want to create a new volume?

Thanks

singlemalt's picture
Submitted by singlemalt on Thu, 06/16/2011 - 3:52pm

You can down a data stripe group and have data on other stripe groups available. see man cvadmin and the verb down. Or mark the stripe as down
in the .cfg file. Obviously you can't mark the meta data stripe group as
down and start the volume. I think your management may not be thinking this
through though. What happens if the irrecoverable lun is the meta data lun?
Or if the file system gets corrupt to the point that you can't start the volume
at all? Wouldn't matter then if you did have multiple copies of the file on
different stripe groups since you can't get the volume to start.

jdub's picture
Submitted by jdub on Fri, 06/17/2011 - 12:46am

Thanks, that helps clarify things a lot.

So there's some isolation to the storage pools in a volume. But creating a new volume with its own metadata lun would add extra redundancy.

singlemalt's picture
Submitted by singlemalt on Fri, 06/17/2011 - 10:27am

jdub wrote:
Thanks, that helps clarify things a lot.

So there's some isolation to the storage pools in a volume. But creating a new volume with its own metadata lun would add extra redundancy./quote

Correct. Other things to keep in mind. If you only have one mdc then that would also be a single point of failure, as would only having one Fibre channel switch.