Re: Pyramid erasure codes and replica hinted recovery

[Loic Dachary <loic@dachary.org>]

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On 13/01/2014 03:35, Kyle Bader wrote:
>> How is it different from what is described above? There must be something I fail to understand.
> 
> No misunderstanding on your part, on second look that does achieve the
> desired placement. Could you please help walk me through the following
> scenarios:
> 
> Can data or local parity chunks that have been lost (erasures) be
> recovered locally, with no inter-dc backfill traffic?

If the primary happens to be located in the same data enter as the lost chunk and the layout is as described previously, then it will be recovered without the need for inter-dc traffic. If the primary is not in the same datacenter, it may be possible to move it to the datacenter where the lost chunk is located. When the primary OSD is lost, another must be chosen. It would be nice to change the primary not only when it is lost but also when doing so helps recovery. 

> Global parity chunks that are lost require reading....6x data or
> global parity chunks (effectively 1x the original write)?

From the point of view of recovery, global parity chunks are treated in the same way as data chunks. If you have RS(6,3,3), you will need to read 6 chunks out of 9 ( 6 data chunks + 3 global parity chunks ) to be able to recover from the loss of 2 or 3 chunks ( data or parity, it does not matter ). In other words, to recover from the loss of more chunks than local parity allows, you need to read 1x the original write. 

> Would placement groups containing a data or local parity chunk that
> have been remapped backfill from the local chunk (member of previous
> acting set)?

David is working on multiple backfill at the moment https://github.com/ceph/ceph/pull/931 and will have a definitive answer. The data flows from the primary OSD to the OSDs supporting the other chunks there is no peer-to-peer communication between the OSDs participating in a placement group. 

Cheers

-- 
Loïc Dachary, Artisan Logiciel Libre


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