Re: Review request : Erasure Code plugin loader implementation

[Loic Dachary <loic@dachary.org>]

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On 19/08/2013 02:01, Sage Weil wrote:
> On Sun, 18 Aug 2013, Loic Dachary wrote:
>> Hi Sage,
>>
>> Unless I misunderstood something ( which is still possible at this stage ;-) decode() is used both for recovery of missing chunks and retrieval of the original buffer. Decoding the M data chunks is a special case of decoding N <= M chunks out of the M+K chunks that were produced by encode(). It can be used to recover parity chunks as well as data chunks.
>>
>> https://github.com/dachary/ceph/blob/wip-4929/doc/dev/osd_internals/erasure-code.rst#erasure-code-library-abstract-api
>>
>>     map<int, buffer> decode(const set<int> &want_to_read, const map<int, buffer> &chunks)
>>
>>     decode chunks to read the content of the want_to_read chunks and return a map associating the chunk number with its decoded content. For instance, in the simplest case M=2,K=1 for an encoded payload of data A and B with parity Z, calling
>>
>>     decode([1,2], { 1 => 'A', 2 => 'B', 3 => 'Z' })
>>     => { 1 => 'A', 2 => 'B' }
>>
>>     If however, the chunk B is to be read but is missing it will be:
>>
>>     decode([2], { 1 => 'A', 3 => 'Z' })
>>     => { 2 => 'B' }
> 
> Ah, I guess this works when some of the chunks contain the original 
> data (as with a parity code).  There are codes that don't work that way, 
> although I suspect we won't use them.
> 
> Regardless, I wonder if we should generalize slightly and have some 
> methods work in terms of (offset,length) of the original stripe to 
> generalize that bit.  Then we would have something like
> 
>      map<int, buffer> transcode(const set<int> &want_to_read, const map<int, 
>             buffer>& chunks);
> 
> to go from chunks -> chunks (as we would want to do with, say, a LRC-like 
> code where we can rebuild some shards from a subset of the other shards).  
> And then also have
> 
>      int decode(const map<int, buffer>& chunks, unsigned offset, 
>          unsigned len, bufferlist *out);

This function would be implemented more or less as:

  set<int> want_to_read = range_to_chunks(offset, len) // compute what chunks must be retrieved
  set<int> available = the up set
  set<int> minimum = minimum_to_decode(want_to_read, available);
  map<int, buffer> available_chunks = retrieve_chunks_from_osds(minimum);
  map<int, buffer> chunks = transcode(want_to_read, available_chunks); // repairs if necessary
  out = bufferptr(concat_chunks(chunks), offset - offset of the first chunk, len)

or do you have something else in mind ?

> 
> that recovers the original data.
> 
> In our case, the read path would use decode, and for recovery we would use 
> transcode.  
> 
> We'd also want to have alternate minimum_to_decode* methods, like
> 
>     virtual set<int> minimum_to_decode(unsigned offset, unsigned len, const 
>          set<int> &available_chunks) = 0;

I also have a convenience wrapper in mind for this but I feel I'm missing something.

Cheers

> 
> What do you think?
> 
> sage
> 
> 
> 
> 
>>
>> Cheers
>>
>> On 18/08/2013 19:34, Sage Weil wrote:
>>> On Sun, 18 Aug 2013, Loic Dachary wrote:
>>>> Hi Ceph,
>>>>
>>>> I've implemented a draft of the Erasure Code plugin loader in the context of http://tracker.ceph.com/issues/5878. It has a trivial unit test and an example plugin. It would be great if someone could do a quick review. The general idea is that the erasure code pool calls something like:
>>>>
>>>> ErasureCodePlugin::factory(&erasure_code, "example", parameters)
>>>>
>>>> as shown at
>>>>
>>>> https://github.com/ceph/ceph/blob/5a2b1d66ae17b78addc14fee68c73985412f3c8c/src/test/osd/TestErasureCode.cc#L28
>>>>
>>>> to get an object implementing the interface
>>>>
>>>> https://github.com/ceph/ceph/blob/5a2b1d66ae17b78addc14fee68c73985412f3c8c/src/osd/ErasureCodeInterface.h
>>>>
>>>> which matches the proposal described at
>>>>
>>>> https://github.com/dachary/ceph/blob/wip-4929/doc/dev/osd_internals/erasure-code.rst#erasure-code-library-abstract-api
>>>>
>>>> The draft is at
>>>>
>>>> https://github.com/ceph/ceph/commit/5a2b1d66ae17b78addc14fee68c73985412f3c8c
>>>>
>>>> Thanks in advance :-)
>>>
>>> I haven't been following this discussion too closely, but taking a look 
>>> now, the first 3 make sense, but
>>>
>>>     virtual map<int, bufferptr> decode(const set<int> &want_to_read, const 
>>> map<int, bufferptr> &chunks) = 0;
>>>
>>> it seems like this one should be more like
>>>
>>>     virtual int decode(const map<int, bufferptr> &chunks, bufferlist *out);
>>>
>>> As in, you'd decode the chunks you have to get the actual data.  If you 
>>> want to get (missing) chunks for recovery, you'd do
>>>
>>>   minimum_to_decode(...);  // see what we need
>>>   <fetch those chunks from other nodes>
>>>   decode(...);   // reconstruct original buffer
>>>   encode(...);   // encode missing chunks from original data
>>>
>>> sage
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>>
>> -- 
>> Lo?c Dachary, Artisan Logiciel Libre
>> All that is necessary for the triumph of evil is that good people do nothing.
>>
>>
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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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