Locally Repairable Codes vs Pyramid

[Loic Dachary <loic@dachary.org>]

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Hi Ceph,

TL;DR: s/pyramid/LRC/ in the implementation of http://pad.ceph.com/p/cdsgiant-pyramid-erasure-code for clarity

Although the term "Pyramid code" has been consistently used in reference to the technique used to reduce the network requirements when repairing from the loss of a single OSD (put simply ;-), I realized today when discussing with Dan Lambright and Xavier Hernandez from glusterfs that I was unclear about what it really means.

As far as I can tell, LRC as described in http://anrg.usc.edu/~maheswaran/Xorbas.pdf suggests an implementation that is close to what has been described during the last CDS. In contrast the pyramid code paper http://research.microsoft.com/pubs/70415/tr-2007-25.pdf suggests a more sophisticated approach which I do not fully understand. 

Instead of hardcoding the word "pyramid" in the pathnames of the implementation it seems better to use "LRC" instead.

Cheers

-- 
Loïc Dachary, Artisan Logiciel Libre


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