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); 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; 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
