Hi Sam, I updated the API description at https://github.com/dachary/ceph/blob/wip-4929/doc/dev/osd_internals/erasure-code.rst#erasure-code-library-abstract-api Please let me know if I missed something. Cheers On 02/07/2013 23:33, Samuel Just wrote: > I think we should be able to cover most cases by adding an interface like: > > set<int> minimum_to_read(const set<int> &want_to_read, const set<int> > &available_chunks); > > which returns the smallest set required to read/rebuild the chunks in > want_to_read given the chunks in available_chunks. Alternately, we > might include a "cost" for reading each chunk like > > set<int> minimum_to_read_with_cost(const set<int> &want_to_read, const > map<int, int> &available) > > which returns the minimum cost set required to read the specified > chunks given a mapping of available chunks to costs. The costs might > allow us to consider the difference between reading local chunks vs > remote chunks. This should be sufficient to cover the read case (esp > the degraded read case) and the repair case. > -Sam > > On Tue, Jul 2, 2013 at 10:14 AM, Atchley, Scott <atchleyes@xxxxxxxx> wrote: >> On Jul 2, 2013, at 10:07 AM, "Atchley, Scott" <atchleyes@xxxxxxxx> wrote: >> >>> On Jul 1, 2013, at 7:00 PM, Loic Dachary <loic@xxxxxxxxxxx> wrote: >>> >>>> Hi, >>>> >>>> Today Sam pointed out that the API for LRC ( Xorbas Hadoop Project Page, Locally Repairable Codes (LRC) http://smahesh.com/HadoopUSC/ for instance ) would need to be different from the one initialy proposed: >>> >>> An interesting video. Not as entertaining as Jim Plank's video. ;-) >>> >>> While Plank's focused on the processor requirements for encoding/decoding, this video focuses on the network and disk I/O requirements. >>> >>>> context(k, m, reed-solomon|...) => context* c >>>> encode(context* c, void* data) => void* chunks[k+m] >>>> decode(context* c, void* chunk[k+m], int* indices_of_erased_chunks) => void* data // erased chunks are >>>> not used >>>> repair(context* c, void* chunk[k+m], int* indices_of_erased_chunks) => void* chunks[k+m] // erased >>>> chunks are rebuilt >>>> >>>> The decode function must allow for partial read: >>>> >>>> decode(context* c, int offset, int length, void* chunk[k+m], int* indices_of_erased_chunks, int* missing_chunks) => void* data >>>> >>>> If there are not enough chunks to recover the desired data range [offset, offset+length) the function returns NULL and sets missing_chunks to the list of chunks that must be retrieved in order to be able to read the desired data. >>>> >>>> If decode is called to read just 1 chunk and it is missing, reed-solomon would return on error and ask for all other chunks to repair. If the underlying library implements LRC, it would ask for a subset of the chunks. >>>> >>>> An implementation allowing only full reads and using jerasure ( which does not do LRC ) requires that offset is always zero, length is the size of the object and returns a copy of indices_of_erased_chunks if there are not enough chunks to rebuild the missing ones. >>>> >>>> Comments are welcome :-) >>> >>> I have loosely followed this discussion and I have not looked closely at the proposed API nor at the jerasure interface. My apologies if this has already been addressed. >>> >>> It is not clear to me from the above proposed API (ignoring the partial read) what it would do. Was the original intent to encode the entire file using k+m blocks irregardless of the file size and of the rados object size? >>> >>> If so, how will you map rados objects to the logical k+m objects and vice versa? >>> >>> If not, then the initial API needed an offset and length (either logical or rados object). >>> >>> I would assume that you would want to operate on rados sized objects. Given a fixed k+m, then you may have more than one set of k+m objects per file. This is ignoring the LRC "local" parity blocks. For example, if the rados object size if 1 MB and k = 10 and m = 4 (as in the Xorbas video), then for a 20 MB file one would need two sets of encoding blocks. The first for objects 1-10 and the second for objects 11-20. >>> >>> Perhaps, this is what the context is above. If so, it should have the logical offset and rados object size, no? >>> >>> I see value in the Xorbas concept and I wonder if the jerasure library can be modified to generate the local parity blocks such that they can be used to generate the global parity blocks. That would be a question for Jim Plank. >> >> The benefits of the Xorbas concept is reduced network and disk I/O for failures while maintaining traditional RS's higher fault-tolerance than 3x replication while using less space. >> >> You can do almost the same thing with jerasure without modifying it at all. Using the values from the Xorbas video, they have k data blocks, m global parity blocks, and 2 local parity blocks (generated from k/2 data blocks) for a total of k+m+2 blocks on disk that can tolerate any m failures. In their example, k = 10 and m = 4. They store 16 blocks for each 10 data blocks. >> >> If you use traditional RS encoding via jerasure and used the same amount of storage (16 blocks for each 10 data blocks), you could encode 3 parity blocks for each 5 data blocks. This would consume 16 data blocks for each 10 data blocks and the fault-tolerance would be variable from 3-6 failures depending on how the failures fell between the two groups of 5 blocks which is higher than the static 4 failures for the Xorbas code. The I/O to recover from a single failure for both schemes is 5 blocks so it is as efficient as Xorbas. On average, it provides more fault-tolerance, but it can be less (four failures from one group of 5 data + 3 parity blocks), but that worst case is the same as 3x replication. >> >> Scott-- >> 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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