[gpfsug-discuss] restripe or not

[Zachary Giles]

Interesting question.. Just some thoughts: Not an expert on restriping
myself:

* Your new storage -- is it the same size, shape, speed as the old storage?
If not, then are you going to add it to the same storage pool, or an
additional storage pool? If additional, restripe is not needed, as you
can't / don't need to restripe across storage pools, the data will be in
one or the other. However, you of course will need to make a policy to
place data correctly.
Of course, if you're going to double your storage and all your new data
will be written to the new disks, then you may be leaving quite a bit of
capacity on the floor.

* mmadddisk man page and normal balancing -- yes, we've seen this
suggestion as well -- that is, that new data will generally fill across the
cluster and eventually fill in the gaps. We didn't restripe on a much
smaller storage pool and it eventually did balance out, however, it was
also a "tier 1" where data is migrated out often. If I were doubling my
primary storage with more of the exact same disks, I'd probably restripe.

* Stopping a restripe -- I'm """ Pretty Sure """ you can stop a restripe
safely with a Ctrl-C. I'm """ Pretty Sure """ we've done that a few times
ourselves with no problem. I remember I was going to restripe something but
the estimates were too high and so I stopped it. I'd feel fairly confident
in doing it, but I don't want to take responsibility for your storage. :)
:)  I don't think there's a need to restripe every hour or anything. If
you're generally balanced at one point, you'd probably continue to be under
normal operation.





On Mon, Nov 24, 2014 at 4:22 PM, Stijn De Weirdt <stijn.deweirdt at ugent.be>
wrote:

> hi all,
>
> we are going to expand an existing filestytem with approx 50% capacity.
> the current filesystem is 75% full.
>
> we are in downtime (for more then just this reason), so we can take the IO
> rebalance hit for a while (say max 48hours).
>
> my questions:
> a. do we really need to rebalance? the mmadddisk page suggest normally
> it's going to be ok, but i never understood that. new data will end up
> mainly on new disks, so wrt to performance, this can't really work out, can
> it?
> b. can we change the priority of rebalancing somehow (fewer nodes taking
> part in the rebalance?)
> c. once we start the rebalance, how save is it to stop with kill or ctrl-c
> (or can we say eg. rebalance 25% now, rest later?)
> (and how often can we do this? eg a daily cron job to restripe at max one
> hour per day, would this cause issue in the long term
>
>
> many thanks,
>
> stijn
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>



-- 
Zach Giles
zgiles at gmail.com
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