Dear Community I am a young engineer (not software or math, please bare with me) with some suggestions regarding erasure codes. I never used Ceph before or any other distributed file system. I stumped upon the suggestion for adding erasure codes to Ceph, as described in this article http://wiki.Ceph.com/01Planning/02Blueprints/Dumpling/Erasure_encoding_as_a_storage_backend first I would like to say great initiative to add erasure codes to Ceph. Ceph needs its own implementation and it have to be done right, I cannot stress this enough, suggested software mentioned in that article would result in very low performance. Why? Reed-Solomon is normally something regarded as being very slow compared to other erasure codes, because the underlying Galois-Field multiplication is slow. Please see video at usenix.org forexplanation. The implementations of Zfec library and other suggested software the others rely on the Vandermonde matrix, a matrix used in in Reed-Solomon erasure codes, a faster approach would be to use the Cauchy-Reed-Solomon implementation. Please see [1,2,3] Although there is something even better, by using the Intel SSE2/3 SIMD instructions it is possible to do the as fast as any other XOR based erasure codes (RaptorQ LT-codes, LDPC etc.). The suggested FECpp lib uses the optimisation but with a relative small Galois-field only 2^8, since Ceph aimes at unlimited scalability increasing the size of the Galois-Field would improve performance [4]. Of course the configured Ceph Object Size and/or Stripe width have to be taken into account. Please see https://www.usenix.org/conference/fast13/screaming-fast-galois-field-arithmetic-using-sse2-extensions The solution Using the GF-Complete open source library [4] to implement Reed-Solomon in Ceph in order to allow Ceph to scale to infinity. James S. Plank the author of GF-complete have developed a library implementing various Reed-Solomon codes called Jerasure. http://web.eecs.utk.edu/~plank/plank/www/software.html Jerasure 2.0 using the GF-complete artimetric based in Intel SSE SIMD instructions, is current in development expected release august 2013. Will be released under the new BSD license. Jerasure 2.0 also supports arbitrary Galois-field sizes 8,16,32,64 or 128 bit. The limit of this implementation would be the processors L2/L3 cache not the underlying arithmetic. Best Regards Martin Flyvbjerg [1] http://web.eecs.utk.edu/~plank/plank/papers/CS-05-569.pdf [2] http://web.eecs.utk.edu/~plank/plank/papers/CS-08-625.pdf [3] http://web.eecs.utk.edu/~plank/plank/papers/FAST-2009.pdf [4] http://web.eecs.utk.edu/~plank/plank/papers/FAST-2013-GF.pdf -- 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
