Re: controlling erasure code chunk size

[Loic Dachary <loic@dachary.org>]

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Hi Andreas,

> For w=(multiple of 8) we could probably skip the (*sizeof(int)) and get the chunksize factor 4 down ... Loic we should check if this is ok with the Jerasure implementation .... I wonder if we should have 'packetsize' as a plugin parameter or we should just adjust the packetsize based on the desired chunk_size to get it close.

You are correct : the packet size is best adapted to the object size (or stripe size) rather than being set once for all. However Sam wants to use a fixed stripe size and we don't need this flexibility right now. 

I don't fully understand the alignment requirements of Jerasure. Since we're using Cauchy because it is the fastest, here is how I understand its alignment constraints. I copied them from the original encode/decode methods found in jerasure into the get_alignment method whithout understanding the details.

* each chunk memory address must be aligned to allow 
https://github.com/ceph/ceph/blob/v0.76/src/osd/ErasureCodePluginJerasure/vectorop.h to be used by https://github.com/ceph/ceph/blob/v0.76/src/osd/ErasureCodePluginJerasure/galois.c#L748 . This is done without reading from get_alignment() because each buffer is created with https://github.com/ceph/ceph/blob/v0.76/src/common/buffer.cc#L519 buffer::create_page_aligned which calls https://github.com/ceph/ceph/blob/v0.76/src/common/buffer.cc#L235 posix_memalign with an alignment of CEPH_PAGE_SIZE which is large enough. It is implicit though and it would be better to explicitly set this constraint.

https://github.com/ceph/ceph/blob/v0.76/src/osd/ErasureCodePluginJerasure/ErasureCodeJerasure.cc#L288

* each chunk size must be a multiple of get_alignment() and in the case of the Cauch techniques it means:
** being a multiple of sizeof(int) (why?)
** being a multiple of LARGEST_VECTOR_WORDSIZE (because https://github.com/ceph/ceph/blob/v0.76/src/osd/ErasureCodePluginJerasure/galois.c#L748)
** being a multiple of k*w*packetsize (because each chunk contains k packets of packets size and each packet is made of words of size w)

I would be grateful if you could explain what the sizeof(int) is about. Also, I understand that k*w*packetsize should be a multiple of LARGEST_VECTOR_WORDSIZE but I don't understand why you would multiply the alignment to achieve this. Is it be enough to if (alignment % LARGEST_VECTOR_WORDSIZE) alignment += alignment % LARGEST_VECTOR_WORDSIZE ? 

Thanks in advance for your patience :-)

-- 
Loïc Dachary, Artisan Logiciel Libre


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