Re: durability model

[Loic Dachary <loic@dachary.org>]

[-- Attachment #1: Type: text/plain, Size: 2422 bytes --]



On 27/05/2014 15:30, Koleos Fuskus wrote:> Hi Kyle,
> Thanks for your answer!
> I have other questions regarding the model. Sorry, I accumulated different doubts and the e-mail is a bit long.
> 
> 1. What will be erasure model unit? Is it a pool or a placement group? I cannot see the best option here yet, probably because my inexperience with Ceph.

A pool. 

> 2. Crush_ruleset for erasured pools: Are these crush-ruleset impacting on the reliability model? Should I define a default crush-ruleset? 

Unless I'm mistaken, the crush ruleset is only relevant to the durability model to the extent that it defines the failure domain. If a ruleset requires that no two shards of a given placement group (i.e. no two shards of a given object since a placement group is a set of object that share the same OSD for a given shard) are on the same host, it will lead to a model that is different from a ruleset that requires that no two shards are in the same datacenter.

> 3. Metadata: Jerasure creates metadata files. How are those files stored in ceph (what kind of redundancy)? Should I care about them? I guess the jerasure metafiles are not important. If all objects stored in a pool share the same erasure-code-k and erasure-code-m, then is easy to re-generate the metadata file. However, if metadata file is lost and the pool contains erasured objects with different k and m, then repairing data will not be possible. Should I ignore this problem? Could you confirm that each pool has the same erasure parameters?

Jerasure does not create a metadata file.

> 4. Number of PG for an erasured-pool. The total PGs in the case of replication is calculated with this formula: Total PGs= (OSDs * 100)/Replicas.
>
> Should I replace Replicas for the stretch factor (storage overhead) in the case of erasure?
> 
> 5.Parameters - Default values:
> erasure-code-k (default 4)
> erasure-code-m (default 2)
> 
> OSD (default 200)
> PG (default 100 x OSD)
> stripes (default ????)


I proposed an update to the documentation to clarify this : https://github.com/ceph/ceph/pull/1856 . Does it look sensible to you ?

Cheers

> 6. Units
> It is hardcoded on the reliable tool:
> KiB = 1000
> MiB = KiB * 1000
> GiB = MiB * 1000
> TiB = GiB * 1000
> PiB = TiB * 1000
> 
> Why KiB is 1000 when should be 1024?

> Best, 
> Koleosfuscus
> 

-- 
Loïc Dachary, Artisan Logiciel Libre


[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 263 bytes --]