Re: CEPH Erasure Encoding + OSD Scalability

[Loic Dachary <loic@dachary.org>]

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On 25/09/2013 20:33, Andreas Joachim Peters wrote:> Yes, sure. I actually thought the same in the meanwhile ...  I have some questions:
> 
> Q: Can/should it stay in the framework of google test's or you would prefer just a plain executable ?
> 

A plain executable would make sense. An simple example from src/test/Makefile.am :

ceph_test_trans_SOURCES = test/test_trans.cc
ceph_test_trans_LDADD = $(LIBOS) $(CEPH_GLOBAL)
bin_DEBUGPROGRAMS += ceph_test_trans


> I have added local parity support to your erasure class adding a new argument: "erasure-code-lp" and
> two new methods:
> 
> localparity_encode(...)
> localparity_decode(...)
> 
> I made a more complex benchmark of (8,2) + 2 local parities (1^2^3^4, 5^6^7^8) which benchmarks performance of encoding/decoding as speed & effective write-latency for three cases (each for liberation & cauchy_good codecs):
> 
> 1 (8,2)
> 2 (8,2,lp=2)
> 3 (8,2,lp=2) + crc32c (blocks)
> 
> and several failure scenarios ... single, double, triple disk failures. Probably the best is if I make all this parameters configurable. 

Great :-) Do you have a public git repository where I could clone this & give it a try ?

> Q: For the local parity implementation .... shall I inherit from your erasure plugin and overwrite the encode/decode method or you would consider a patch to the original class?

It is a perfect timing for a patch to the original class.

> I have also a 128-bit XOR implementation for the local parities. This will work with new gcc's & clang compilers ... 
> 
> Q: Which compilers/platforms are supported by CEPH? Is there a minimal GCC version?

You can see all supported platforms here:

http://ceph.com/gitbuilder.cgi

I don't think the GCC version shows in the logs but you can probably figure it out from the corresponding distribution. 

> Q: is there some policy restricting comments within code? In general I see very few or no comments within the code ..

:-) The mon code tends to be more heavily commented than the osd code (IMO) but I'm not aware of any policy. When I feel the need to comment, I write a unit test. If the unit test is difficult, I tend to comment to clarify its purpose. The problem with comments is that they quickly become obsolete and/or misleading. That being said, I don't think anyone will object if you heavily comment your code.

Cheers

> Cheers Andreas.
> 
> 
> 
> 

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


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