Hello Loic, Dimakis (one of the authors of xorbas) is talking about coefficients because they want to find a way to reduce the storage overhead used with LRC. In the simple case used in Fig. 2, a RS (k=10, m=4) has 14/10 storage overhead but when using LRC, the overhead increases to 17/10 because you also need to store s1, s2 and s3. Basically, the idea is to find specific coefficients c1..c10 that permit to obtain s3 through s1 and s2. In other words, get some s1 and s2 that when xored together give s3. If you find such coefficients, you don't need to store s3 and the storage overhead of LRC is 1.6x instead of 1.7x. Dimakis said that for the Reed Solomon implementation used in HDFS RAID they can simple set all coefficients with value '1' and use xor. This cannot be the case of the Reed Solomon implemented by you (I understood is the jerasure library by Plank) but that I am not sure. I guess we need the help of a mathematician or at least check and compare both implementations. Finally, apparently for xorbas they only implemented the configuration RS(10,4) and not other combinations. Unfortunately, the wiki page of the project is empty http://wiki.apache.org/hadoop/ErasureCode and the main page says 'erasure coding under development'. I recommend you to watch the xorbas presentation video http://smahesh.com/HadoopUSC/ (a very clear explanation of xorbas) and use the Dimakis wiki page to check the large collection of paper they have: http://storagewiki.ece.utexas.edu/ Best, koleosfuscus ________________________________________________________________ "My reply is: the software has no known bugs, therefore it has not been updated." Wietse Venema On Sun, Jun 29, 2014 at 11:30 AM, Loic Dachary <loic@xxxxxxxxxxx> wrote: > Hi Andreas, > > In http://anrg.usc.edu/~maheswaran/Xorbas.pdf I get the idea of computing local coding chunks the way it is implemented in https://github.com/ceph/ceph/pull/1921 (i.e. delegating encoding / decoding to other plugins). However, there are theoretical aspects of the paper that I do not understand and I'm hoping you can shed some light on it. In particular, I don't know what "coefficients" are about. For instance in the context of Figure 2 caption : "The main theoretical challenge is to choose the coeffi cients c(i) to maximize the fault tolerance of the code." > > Would you recommend a paper to read to better understand this ? Also I'd like to understand what "coefficients" mean in the context of jerasure or if they do not apply. > > Thanks for you help :-) > > -- > 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
