Hi koleofuscus, Thanks for the explanation : it is very conforting to know that you understand this :-) At the risk of being thick, I must say that the very notion of "coefficient" eludes me. What are they ? Cheers On 29/06/2014 20:38, Koleos Fuscus wrote: > 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 >> -- Loïc Dachary, Artisan Logiciel Libre
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