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. ... For the checksumming ... I saw that there is the check if CRC32C is supported, but I was looking for a generic routine like: crc32c_t crc32c(void* buffer, off_t lenght) which internally selects either the HW accelerated or SW implementation. Mayby you have this in some other source file. 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 > > -- 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
