On 22/08/2013 23:42, Andreas-Joachim Peters wrote:> Hi Loic, > sorry for the late reply, I was on vacation ... you are right, I did a > simple logical mistake since I assumed you loose only the data stripes but > never the parity stripes which is a very wrong assumption. > > So for testing you probably could just implement (2+1) and then move to > jerasure or dual parity (4+2) where you build horizontal and diagonal > parities. >
Hi Andreas, That's what I did :-) It would be great if you could review the proposed implementation at https://github.com/ceph/ceph/pull/518/files . I'll keep working on https://github.com/dachary/ceph/commit/83845a66ae1cba63c122c0ef7658b97b474c2bd2 tomorrow to create the jerasure plugin but it's not ready for review yet. Cheers > Cheers Andreas. > > > > > > On Mon, Aug 19, 2013 at 12:35 PM, Loic Dachary <[email protected] > <mailto:[email protected]>> wrote: > > Hi Andreas, > > Trying to write minimal code as you suggested, for an example plugin. My > first attempt at writing an erasure coding function. I don't get how you can > rebuild P1 + A from P2 + B + C. I must be missing something obvious :-) > > Cheers > > On 07/07/2013 23:04, Andreas Joachim Peters wrote: > > > > Hi Loic, > > I don't think there is a better generic implementation. Just made a > benchmark .. the Jerasure library with algorithm 'cauchy_good' gives 1.1 GB/s > (Xeon 2.27 GHz) on a single core for a 4+2 encoding w=32. Just to give a > feeling if you do 10+4 it is 300 MB/s .... there is a specialized > implementation in QFS (Hadoop in C++) for (M+3) ... for curiosity I will make > a benchmark with this to compare with Jerasure ... > > > > In any case I would do an optimized implementation for 3+2 which would > be probably the most performant implementation having the same reliability > like standard 3-fold replication in CEPH using only 53% of the space. > > > > 3+2 is trivial since you encode (A,B,C) with only two parity operations > > P1 = A^B > > P2 = B^C > > and reconstruct with one or two parity operations: > > A = P1^B > > B = P1^A > > B = P2^C > > C = P2^B > > aso. > > > > You can write this as a simple loop using advanced vector extensions on > Intel (AVX). I can paste a benchmark tomorrow. > > > > Considering the crc32c-intel code you added ... I would provide a > function which provides a crc32c checksum and detects if it can do it using > SSE4.2 or implements just the standard algorithm e.g if you run in a virtual > machine you need this emulation ... > > > > Cheers Andreas. > > ________________________________________ > > From: Loic Dachary [[email protected] <mailto:[email protected]>] > > Sent: 06 July 2013 22:47 > > To: Andreas Joachim Peters > > Cc: [email protected] <mailto:[email protected]> > > Subject: Re: CEPH Erasure Encoding + OSD Scalability > > > > Hi Andreas, > > > > Since it looks like we're going to use jerasure-1.2, we will be able to > try (C)RS using > > > > https://github.com/tsuraan/Jerasure/blob/master/src/cauchy.c > > https://github.com/tsuraan/Jerasure/blob/master/src/cauchy.h > > > > Do you know of a better / faster implementation ? Is there a tradeoff > between (C)RS and RS ? > > > > Cheers > > > > On 06/07/2013 15:43, Andreas-Joachim Peters wrote: > >> HI Loic, > >> (C)RS stands for the Cauchy Reed-Solomon codes which are based on pure > parity operations, while the standard Reed-Solomon codes need more > multiplications and are slower. > >> > >> Considering the checksumming ... for comparison the CRC32 code from > libz run's on a 8-core Xeon at ~730 MB/s for small block sizes while SSE4.2 > CRC32C checksum run's at ~2GByte/s. > >> > >> Cheers Andreas. > >> > >> > >> > >> > >> On Fri, Jul 5, 2013 at 11:23 PM, Loic Dachary <[email protected] > <mailto:[email protected]> <mailto:[email protected] > <mailto:[email protected]>>> wrote: > >> > >> Hi Andreas, > >> > >> On 04/07/2013 23:01, Andreas Joachim Peters wrote:> Hi Loic, > >> > thanks for the responses! > >> > > >> > Maybe this is useful for your erasure code discussion: > >> > > >> > as an example in our RS implementation we chunk a data block of > e.g. 4M into 4 data chunks of 1M. Then we create a 2 parity chunks. > >> > > >> > Data & parity chunks are split into 4k blocks and these 4k > blocks get a CRC32C block checksum each (SSE4.2 CPU extension => MIT library > or BTRFS). This creates 0.1% volume overhead (4 bytes per 4096 bytes) - > nothing compared to the parity overhead ... > >> > > >> > You can now easily detect data corruption using the local > checksums and avoid to read any parity information and (C)RS decoding if > there is no corruption detected. Moreover CRC32C computation is distributed > over several (in this case 4) machines while (C)RS decoding would run on a > single machine where you assemble a block ... and CRC32C is faster than (C)RS > decoding (with SSE4.2) ... > >> > >> What does (C)RS mean ? (C)Reed-Solomon ? > >> > >> > In our case we write this checksum information separate from the > original data ... while in a block-based storage like CEPH it would be > probably inlined in the data chunk. > >> > If an OSD detects to run on BRTFS or ZFS one could disable > automatically the CRC32C code. > >> > >> Nice. I did not know that was built-in :-) > >> > https://github.com/dachary/ceph/blob/wip-4929/doc/dev/osd_internals/erasure-code.rst#scrubbing > >> > >> > (wouldn't CRC32C be also useful for normal CEPH block > replication? ) > >> > >> I don't know the details of scrubbing but it seems CRC is already > used by deep scrubbing > >> > >> https://github.com/ceph/ceph/blob/master/src/osd/PG.cc#L2731 > >> > >> Cheers > >> > >> > As far as I know with the RS CODEC we use you can either miss > stripes (data =0) in the decoding process but you cannot inject corrupted > stripes into the decoding process, so the block checksumming is important. > >> > > >> > Cheers Andreas. > >> > >> -- > >> 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. > > > > -- > > To unsubscribe from this list: send the line "unsubscribe ceph-devel" in > > the body of a message to [email protected] > <mailto:[email protected]> > > 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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