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
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