dev/osd_internals,src/osd: add erasure_coding.rst and PGBackend.h

Signed-off-by: Samuel Just <sam.just@inktank.com>
M+K means M data + K parity and implies M chunks necessary to recover

Signed-off-by: Loic Dachary <loic@dachary.org>
fix typo

Signed-off-by: Loic Dachary <loic@dachary.org>
Add links to backfill related tracker items.

Signed-off-by: Loic Dachary <loic@dachary.org>
Add links to recovery related tracker items.

Signed-off-by: Loic Dachary <loic@dachary.org>
Add links to peering related tracker items.

Signed-off-by: Loic Dachary <loic@dachary.org>
Add links to scrub related tracker items.

Signed-off-by: Loic Dachary <loic@dachary.org>
Add links to OSD internals related tracker items.

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #482 from dachary/wip-erasure-coding-doc

Links to newly created tracker items, typo, M+K convention
document the write / read path for erasure encoding

Explains how objects are stored and used in erasure coded pools. It is
the result of discussions that occured on the ceph-devel mailing list
around june 2013. The rationale behind each change can be found in the
archive of the mailing list. For instance, the coding of the chunk
number with the object or the decision to decode using any K chunks
instead of trying to fetch the data chunks when possible because it
would allow simple concatenation when systematic codes are used.

http://tracker.ceph.com/issues/4929 refs #4929

Signed-off-by: Loic Dachary <loic@dachary.org>
rearrange the documentation to be inserted and maintained in master

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #493 from dachary/wip-erasure-coding-doc

rearrange erasure code documents

Reviewed-by: Samuel Just <sam.just@inktank.com>
Merge branch 'wip-erasure-coded-doc'