Hi Loic, > to recover D6 which is in rack 2 it may be necessary to use P2 from rack 1 Because D6 can be recovered from P3 as well, P2 is not necessarily used to recover D6. However, when D6 is recovered from P3, D5 which is in rack 1 must be read. Therefore, I believe Ceph-LRC is more efficient when network resources is poor such as in case of Geo-Replication. SHEC should be used in a single data center that has enough network resources. Best regards, Takeshi Miyamae -----Original Message----- From: Loic Dachary [mailto:loic@xxxxxxxxxxx] Sent: Tuesday, May 19, 2015 3:39 PM To: Miyamae, Takeshi/宮前 剛 Cc: Ceph Development Subject: What crush ruleset for a given SHEC configuration ? Hi Takeshi, In the context of http://ceph.com/docs/master/rados/operations/erasure-code-shec/ it would be useful to have a more detailed explanation of why SHEC is more efficient during recovery (in the introduction). Am I correct to assume that SHEC does not provide a way to control the locality of the chunks ? For instance in the following scenario: rack 1 has 10 OSDs rack 2 has 10 OSDs a crush ruleset is made to provide 15 OSDs with 7 in the first rack, 8 in the last rack: the first 7 are in rack 1, the last 8 in rack 2. When SHEC is used with such a crush ruleset, it cannot guarantee that the loss of one chunk in rack 2 can always be recovered with chunks from rack 2. When reading at figure 3 of https://wiki.ceph.com/Planning/Blueprints/Hammer/Shingled_Erasure_Code_%28SHEC%29 with D1 to D5, P1 and P2 in rack 1 and D6 to D10, P3, P4, P5 in rack 2, my understanding is that to recover D6 which is in rack 2 it may be necessary to use P2 from rack 1. And to recover D5 which is in rack 1 it may be necessary to use P3 from rack 2. Maybe I'm missing something ? Thanks in advance for your explanations :-) Cheers -- Loïc Dachary, Artisan Logiciel Libre ��.n��������+%������w��{.n����z��u���ܨ}���Ơz�j:+v�����w����ޙ��&�)ߡ�a����z�ޗ���ݢj��w�f
