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 > -----Original Message----- > From: Atchley, Scott [mailto:atchleyes@xxxxxxxx] > Sent: Wednesday, July 03, 2013 4:53 AM > To: Paul Von-Stamwitz > Cc: Samuel Just; Loic Dachary; Ceph Development > Subject: Re: Erasure coding library API > > 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
