Great info! Many thanks!
Tom2015-03-25 13:30 GMT+01:00 Loic Dachary <loic@xxxxxxxxxxx>:
Hi Tom,
On 25/03/2015 11:31, Tom Verdaat wrote:> Hi guys,
>
> We've got a very small Ceph cluster (3 hosts, 5 OSD's each for cold data) that we intend to grow later on as more storage is needed. We would very much like to use Erasure Coding for some pools but are facing some challenges regarding the optimal initial profile “replication” settings given the limited number of initial hosts that we can use to spread the chunks. Could somebody please help me with the following questions?
>
> 1.
>
> Suppose we initially use replication in stead of erasure. Can we convert a replicated pool to an erasure coded pool later on?
What you would do is create an erasure coded pool later and have the initial replicated pool as a cache in front of it.
http://docs.ceph.com/docs/master/rados/operations/cache-tiering/
Objects from the replicated pool will move to the erasure coded pool if they are not used and it will save space. You don't need to create the erasure coded pool on your small cluster. You can do it when it grows larger or when it becomes full.
> 2.
>
> Will Ceph gain the ability to change the K and N values for an existing pool in the near future?
I don't think so.
> 3.
>
> Can the failure domain be changed for an existing pool? E.g. can we start with failure domain OSD and then switch it to Host after adding more hosts?
The failure domain, although listed in the erasure code profile for convenience, really belongs to the crush ruleset applied to the pool. It can therefore be changed after the pool is created. It is likely to result in objects moving a lot during the transition but it should work fine otherwise.
> 4.
>
> Where can I find a good comparison of the available erasure code plugins that allows me to properly decide which one suits are needs best?
In a nutshell, jerasure is flexible and is likely to be what you want, isa computes faster than jerasure but only works on intel processors (note however that the erasure code computation does not make a significant difference overall), lrc and shec (to be published in hammer) minimize network usage during recovery but uses more space than jerasure or isa.
Cheers
> Many thanks for your help!
>
> Tom
>
>
>
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--
Loïc Dachary, Artisan Logiciel Libre
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