Hi Loic, the XOR+ is more generic with Jerasure because the ISA library does not create an invertible matrix for M>4 in all cases. I measured a tiny advantage for the Intel library encoding/decoding speed for given K/M values and given hardware, but Dan did some measurements in a real cluster and you cannot see any difference on a global scale between a jerasure or isa configuration (8,4) with our hardware. The EC computation does not define the base line performance in that case. So, this is more a general question ... but I am sure it is simpler to implement that within a plug-in rather than on top of N plug-ins ... Cheers Andreas. ________________________________________ From: Loic Dachary [loic@xxxxxxxxxxx] Sent: 20 March 2015 13:42 To: Andreas Joachim Peters Cc: ceph-devel@xxxxxxxxxxxxxxx Subject: Re: Hitchhiker erasure code On 20/03/2015 13:37, Andreas Joachim Peters wrote: > Hi Loic, > I looked at that some time ago. > > Table 1 in the paper says it all: > > If you care about decoding and reconstruction of data it gives a good improvement. > If you care mainly about encoding speed, it is not the optimal choice (+72.1%). > > The algorithm optimizes the reconstruction of data units. This is relevant if your read-size is typically smaller than the block-size e.g. you encode 4 MB objects and you read 4kb pages. With normal EC you get a read amplification of K*4k if a data stripe is down, while with hitchhiker you get only 2/3 of that traffic in case of (10,4). > > The most interesting to implement is probably Hitchhiker-XOR+, which you have to combine with a Vandermonde matrix, it requires that the first parity is just the xor of all data chunks. > > So, yes, there is certainly a benefit in implementing that compared to other approaches (Xorbas,LRC) since it does not involve a space overhead and opens the door to use larger K values and save space! > That sounds appealing :-) Do you think it would be more relevant to implement this as an additional Ceph plugin ? Or as a new jerasure technique ? -- Loïc Dachary, Artisan Logiciel Libre -- 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
