On 05/07/2013 14:13, Atchley, Scott wrote:> 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? Hi Scott, Does that mean that if I give the chunks A + B + C to decode() and B is corrupted but A and C are ok it will return an incorrectly decoded content ? I'm curious to know the answer but I don't think it is an actual problem. A corrupted chunk would mean that the underlying file system corrupted the content of one of its file. I can't remember the last time I saw that happen ;-) Cheers > > 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. >> > -- Loïc Dachary, Artisan Logiciel Libre All that is necessary for the triumph of evil is that good people do nothing.
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