Hi Sage, I created "erasure code : convenience functions to code / decode" http://tracker.ceph.com/issues/6064 to implement the suggested functions. Please let me know if this should be merged with another task. Cheers On 19/08/2013 17:06, Loic Dachary wrote: > > > On 19/08/2013 02:01, Sage Weil wrote: >> On Sun, 18 Aug 2013, Loic Dachary wrote: >>> Hi Sage, >>> >>> Unless I misunderstood something ( which is still possible at this stage ;-) decode() is used both for recovery of missing chunks and retrieval of the original buffer. Decoding the M data chunks is a special case of decoding N <= M chunks out of the M+K chunks that were produced by encode(). It can be used to recover parity chunks as well as data chunks. >>> >>> https://github.com/dachary/ceph/blob/wip-4929/doc/dev/osd_internals/erasure-code.rst#erasure-code-library-abstract-api >>> >>> map<int, buffer> decode(const set<int> &want_to_read, const map<int, buffer> &chunks) >>> >>> decode chunks to read the content of the want_to_read chunks and return a map associating the chunk number with its decoded content. For instance, in the simplest case M=2,K=1 for an encoded payload of data A and B with parity Z, calling >>> >>> decode([1,2], { 1 => 'A', 2 => 'B', 3 => 'Z' }) >>> => { 1 => 'A', 2 => 'B' } >>> >>> If however, the chunk B is to be read but is missing it will be: >>> >>> decode([2], { 1 => 'A', 3 => 'Z' }) >>> => { 2 => 'B' } >> >> Ah, I guess this works when some of the chunks contain the original >> data (as with a parity code). There are codes that don't work that way, >> although I suspect we won't use them. >> >> Regardless, I wonder if we should generalize slightly and have some >> methods work in terms of (offset,length) of the original stripe to >> generalize that bit. Then we would have something like >> >> map<int, buffer> transcode(const set<int> &want_to_read, const map<int, >> buffer>& chunks); >> >> to go from chunks -> chunks (as we would want to do with, say, a LRC-like >> code where we can rebuild some shards from a subset of the other shards). >> And then also have >> >> int decode(const map<int, buffer>& chunks, unsigned offset, >> unsigned len, bufferlist *out); > > This function would be implemented more or less as: > > set<int> want_to_read = range_to_chunks(offset, len) // compute what chunks must be retrieved > set<int> available = the up set > set<int> minimum = minimum_to_decode(want_to_read, available); > map<int, buffer> available_chunks = retrieve_chunks_from_osds(minimum); > map<int, buffer> chunks = transcode(want_to_read, available_chunks); // repairs if necessary > out = bufferptr(concat_chunks(chunks), offset - offset of the first chunk, len) > > or do you have something else in mind ? > >> >> that recovers the original data. >> >> In our case, the read path would use decode, and for recovery we would use >> transcode. >> >> We'd also want to have alternate minimum_to_decode* methods, like >> >> virtual set<int> minimum_to_decode(unsigned offset, unsigned len, const >> set<int> &available_chunks) = 0; > > I also have a convenience wrapper in mind for this but I feel I'm missing something. > > Cheers > >> >> What do you think? >> >> sage >> >> >> >> >>> >>> Cheers >>> >>> On 18/08/2013 19:34, Sage Weil wrote: >>>> On Sun, 18 Aug 2013, Loic Dachary wrote: >>>>> Hi Ceph, >>>>> >>>>> I've implemented a draft of the Erasure Code plugin loader in the context of http://tracker.ceph.com/issues/5878. It has a trivial unit test and an example plugin. It would be great if someone could do a quick review. The general idea is that the erasure code pool calls something like: >>>>> >>>>> ErasureCodePlugin::factory(&erasure_code, "example", parameters) >>>>> >>>>> as shown at >>>>> >>>>> https://github.com/ceph/ceph/blob/5a2b1d66ae17b78addc14fee68c73985412f3c8c/src/test/osd/TestErasureCode.cc#L28 >>>>> >>>>> to get an object implementing the interface >>>>> >>>>> https://github.com/ceph/ceph/blob/5a2b1d66ae17b78addc14fee68c73985412f3c8c/src/osd/ErasureCodeInterface.h >>>>> >>>>> which matches the proposal described at >>>>> >>>>> https://github.com/dachary/ceph/blob/wip-4929/doc/dev/osd_internals/erasure-code.rst#erasure-code-library-abstract-api >>>>> >>>>> The draft is at >>>>> >>>>> https://github.com/ceph/ceph/commit/5a2b1d66ae17b78addc14fee68c73985412f3c8c >>>>> >>>>> Thanks in advance :-) >>>> >>>> I haven't been following this discussion too closely, but taking a look >>>> now, the first 3 make sense, but >>>> >>>> virtual map<int, bufferptr> decode(const set<int> &want_to_read, const >>>> map<int, bufferptr> &chunks) = 0; >>>> >>>> it seems like this one should be more like >>>> >>>> virtual int decode(const map<int, bufferptr> &chunks, bufferlist *out); >>>> >>>> As in, you'd decode the chunks you have to get the actual data. If you >>>> want to get (missing) chunks for recovery, you'd do >>>> >>>> minimum_to_decode(...); // see what we need >>>> <fetch those chunks from other nodes> >>>> decode(...); // reconstruct original buffer >>>> encode(...); // encode missing chunks from original data >>>> >>>> sage >>>> -- >>>> To unsubscribe from this list: send the line "unsubscribe ceph-devel" in >>>> the body of a message to majordomo@xxxxxxxxxxxxxxx >>>> More majordomo info at http://vger.kernel.org/majordomo-info.html >>>> >>> >>> -- >>> Lo?c Dachary, Artisan Logiciel Libre >>> All that is necessary for the triumph of evil is that good people do nothing. >>> >>> >> -- >> To unsubscribe from this list: send the line "unsubscribe ceph-devel" in >> the body of a message to majordomo@xxxxxxxxxxxxxxx >> More majordomo info at http://vger.kernel.org/majordomo-info.html >> > -- Loïc Dachary, Artisan Logiciel Libre All that is necessary for the triumph of evil is that good people do nothing.
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