Re: Comments on Ceph distributed parity implementation

[Loic Dachary <loic@dachary.org>]

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On 06/21/2013 03:23 AM, Paul Von-Stamwitz wrote:
> 
> On 06/20/2013 11:26 PM, Loic Dachary wrote:
>>
>> I wrote down a short description of the read/write path I plan to
>> implement in ceph :
> https://github.com/dachary/ceph/blob/wip-4929/doc/dev/osd_internals/erasure-code.rst.
>> A quick look at the drawings will hopefully give you an idea. Each OSD is > a disk connected to the others over the network. Although I chose K+M = 5 > I suspect the most common use case will be around K+M = 7+3 = 10
> 
> Hi Loic,
> 
> A couple questions regarding your proposal:
> 
> Where would encode/decode occur? Client or OSD? Previous discussions seemed to want it at the OSD level. Your proposal seems that it could be either.

I think it should be in the OSD. Although it would be possible to implement it in the client, it would have to be implemented in the OSD anyway for scrubbing. Therefore I think it is simpler to implement it in the OSD as a first step.

> 
> What about partial reads? For example, if only 'H' was requested, would you decode the entire object first? 

Unless you see a good reason to implement this optimization from the start, I think the entire object would be decoded.

> Or, read from OSD1 and reconstruct on error?

I think that's what we want, ultimately. And also to encode / decode large objects (1GB for instance) as a sequence of independant regions (4MB for instance or smaller). 

I updated https://github.com/dachary/ceph/blob/wip-4929/doc/dev/osd_internals/erasure-code.rst to reflect our discussion.

The first, simplest implementation is likely to be fit to use with RGW and probably too slow to use with RBD. Do you think we should try to optimize for RBD right now ? 

Cheers

> 
> Thanks,
> Paul
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-- 
Loïc Dachary, Artisan Logiciel Libre
All that is necessary for the triumph of evil is that good people do nothing.



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