On 07/11/2014 15:37, eric mourgaya wrote:> Hi, > > " In erasure coding pool, how do we know which OSDs keeping the data chunk and which one the keep the encoding chunk?" > > There was this question yesterday on ceph irc channel on erasure code. http://ceph.com/docs/giant/dev/erasure-coded-pool/, > we really have a difference between k and m chunks (supposing that m is the number of split and k the number of code chunk), data is splitting in m chunk, and we generate k chunks code based on the previous m chunks. So these chunks are differents, right? http://ceph.com/docs/master/rados/operations/erasure-code-profile/ Yes (except for the fact that k are the data chunks and m the coding chunks). > 1) first in all documentations about erasure, it says that the number of coding is greater than the number of data splitted , Which documentation writes that ? It is usually not the case and if that's in Ceph documentation it would be good to fix it. > but seeing https://wiki.ceph.com/Planning/Blueprints/Dumpling/Erasure_encoding_as_a_storage_backend, I see that the coding chunk number is not greater than the number of splits. What is the goal, reducing the used space? > > 2) We allow the loss of k chunks between (n+k) chunks, > What's happening when you lost a chunk, does ceph rebuilt it somewhere on another osd (not on the n+k-1 previous one)? When you try to read and one chunk is missing, it is rebuilt on the fly. > 3)so is it important to known where are the chunks of code to make a failure rules? Yes. As for replicated pools, choosing the right failure domain is an important architectural decision. > 4) According to these following rules in crush map the erasure code don't take care about failure domain rule. Right? ie the ruletset of the pool don't matter, right? The erasure code encoding / decoding functions does not know about the failure domain. But each erasure coded plugin has a function that creates a sensible crush ruleset based on the parameters it is provided. And the operator can change the failure domain with the ruleset-failure-domain parameters. See the documentation of the jerasure plugin for example: http://ceph.com/docs/master/rados/operations/erasure-code-jerasure/ > let 's take an example: my failure domain is composed by 3 rooms, so usually, in a pool with size equal to 3 we have a replicate in each room. But in erasure coding rule, we don't have this, does the rule applied only on the chunk that contains the code? > > rule replicated_ruleset { > ruleset 0 > type replicated > min_size 1 > max_size 10 > step take default > step chooseleaf firstn 0 type room > step emit > } > > rule erasure-code { > ruleset 1 > type erasure > min_size 3 > max_size 20 > step set_chooseleaf_tries 5 > step take default > step chooseleaf indep 0 type host > step emit > } > > > 5) what do you think about something like : > > rule room_erasure-code { > ruleset 1 > type erasure > min_size 3 > max_size 20 > step set_chooseleaf_tries 5 > step take default > step chooseleaf indep 0 type room > step emit > } > > > And an erasure code with (m=3, and k=2). > and does this settings available with: > > ceph osd erasure-code-profile set failprofile k=2 m=3 > ceph osd erasure-code-profile set failprofile ruleset-root=room_erasure-code > and I can lost 2 room without problem, right? > > I would like to add a summary of your answer in documentation, would you help me on this? > > -- > Eric Mourgaya, > > > Respectons la planete! > Luttons contre la mediocrite! Faisons ça ;-) Cheers -- Loïc Dachary, Artisan Logiciel Libre
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