Loic, Erasure codes take what ever you give them. You need to verify the chunk before using it. Perhaps storing the checksum in the metadata/context that describes the parity object? Scott On Jul 4, 2013, at 9:24 AM, Loic Dachary <loic@xxxxxxxxxxx> wrote: > Hi, > > I was thinking about scrubbing of erasure coded chunks and realized I don't know the answer to this very simple question : what happens when a chunk is corrupted ? I.e. if AB is coded with 2+1 into A + B ( data ) + Z (parity ) and Z is replaced with Q. Would reed-solomon ignore/discard the corrupted chunk ? If that's the case I think it slightly changes what the API should be. > > Cheers > > On 04/07/2013 05:06, Paul Von-Stamwitz wrote: >> Scott, et al. >> >> Here is an interesting paper from Usenix HotStorage Conference which provides local codes without additional capacity overhead. >> >> Check it out. (abstract with links to paper and slides) >> https://www.usenix.org/conference/hotstorage13/solution-network-challenges-data-recovery-erasure-coded-distributed-storage >> >> Cheers, >> pvs >> >>> On Jul 3, 2013, at 11:19 AM, Paul Von-Stamwitz wrote: >>> >>> Hi Scott, >>> >>> Point taken. >>> >>> I was thinking about Loic's decode description where k+m was requested and >>> data was decoded when k blocks were received. But he was referring to full >>> stripe reads where all the memory is allocated. >>> >>> Degraded reads and block repair are a different matter. >>> >>> pvs >>> >>>> On Jul 3, 2013, at 4:53 AM Scott Atchley wrote: >>>> >>>> On Jul 2, 2013, at 10:12 PM, Paul Von-Stamwitz >>>> <PVonStamwitz@xxxxxxxxxxxxxx> wrote: >>>> >>>>> Scott, >>>>> >>>>> You make a good point comparing (5/3) RS with Xorbas, but a small nit: >>>>> >>>>> "The I/O to recover from a single failure for both schemes is 5 blocks >>>> so it is as efficient as Xorbas." >>>>> >>>>> Maybe not. You would probably issue I/O to all the remaining 7 blocks >>> to >>>> cover for the possibility of double errors. The time to reconstruct >>> would >>>> be about the same, but there could be more disk and network I/O. (LRC >>> will >>>> need to issue I/O to the rest of the global stripe if it detected >>> multiple >>>> local errors.) >>>> >>>> Why would you request more than five? If one cannot be read, request >>>> another. >>>> >>>> Also, I am not sure that you want to request five at once since it will >>>> lead to spikes in network traffic and require memory for all five blocks. >>>> You will need at least two buffers. Request the first two and start the >>>> decoding. You may want a third buffer to overlap the decoding of the >>>> current block with the communication for the next block. It may be that >>>> the decode time is less than the communication and, in that case, you >>> will >>>> want to request all of the blocks at once. >>>> >>>>> What I like about Xorbas is that it is an extension of a (x,y) RS. You >>>> can start with traditional RS. If degraded reads and repaired blocks are >>>> causing a problem, you can add the LRC. If capacity is an issue, you can >>>> take it out. >>>> >>>> I like it too and Microsoft has something similar with Pyramid codes. >>> That >>>> said, my example using traditional RS can provide more fault-tolerance >>> on >>>> average given the same amount of storage overhead. >>>> >>>>> >>>>> Best, >>>>> Paul >>>>> >>>>> On Tue, Jul 2, 2013 at 2:33 PM, 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 >>>>>> -- >>>>>> 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 >> >> -- >> 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. > -- 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
