On 08/07/2013 12:00, Andreas Joachim Peters wrote: > Hi Loic, > > I did the two mentioned benchmarks: > > QFS (m+3) code run's at 300 MB/s ... not worthy (jerasure 390 MB/s). > > I made a quick (3+2) encoding benchmark and this encodes ~ 3 GB/s. >
Hi Andreas, It looks like the simplest and fastest implementation there is :-) I understand it only addresses 3+2 but it would make for a fine default implementation / example for the erasure coding plugin implementing the proposed API https://github.com/dachary/ceph/blob/wip-4929/doc/dev/osd_internals/erasure-code.rst#erasure-code-library-abstract-api Cheers > Cheers Andreas. > > ________________________________________ > From: Sage Weil [[email protected]] > Sent: 08 July 2013 05:37 > To: Andreas Joachim Peters > Cc: Loic Dachary; [email protected] > Subject: RE: CEPH Erasure Encoding + OSD Scalability > > On Sun, 7 Jul 2013, Andreas Joachim Peters wrote: >> 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 ... > > The current code in master will do this detection by checking the cpu > features; see > > https://github.com/ceph/ceph/blob/master/src/common/crc32c-intel.c#L74 > > If there is a better way to do this, I'd love to hear about it. gcc 4.8 > just added a bunch of built-in functions to do this stuff cleanly, but > it'll be quite a while before all of our build targets are on 4.8 or > later. > > sage > > >> >> Cheers Andreas. >> ________________________________________ >> From: Loic Dachary [[email protected]] >> Sent: 06 July 2013 22:47 >> To: Andreas Joachim Peters >> Cc: [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]>> 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. >>> >>> -- >>> Lo?c Dachary, Artisan Logiciel Libre >>> All that is necessary for the triumph of evil is that good people do >>> nothing. >>> >>> >> >> -- >> Lo?c 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] >> 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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