common: simpler erasure code technique

Change the default technique from Cauchy to ReedSolomon. Although it is
less efficient the alignment constraints are more intuitive.

Signed-off-by: Loic Dachary <loic@dachary.org>
mon: add erasure-code pg_pool_t::stripe_width

Contains the actual stripe size used by erasure coded pools. It is
initialized to zero by default and has to be explicitly set by erasure
coded pools. get/set methods are added inline.

Signed-off-by: Loic Dachary <loic@dachary.org>
unittests: update osdmaptools with stripe_width

stripe_width 0 now shows on every osd dump.

Signed-off-by: Loic Dachary <loic@dachary.org>
common: add osd_pool_erasure_code_stripe_width

and document it.

Signed-off-by: Loic Dachary <loic@dachary.org>
mon: osd pool create sets pg_pool_t::stripe_width

It does nothing if the pool is replicated. Otherwise it uses
osd_pool_erasure_code_stripe_width as the desired stripe width and run
it by get_chunk_size() on the erasure code plugin to get the actual
stripe_width. It will always be >= 0 to the desired stripe_width, padded
to match the alignment constraints imposed by the erasure code plugin.

Signed-off-by: Loic Dachary <loic@dachary.org>
mon: test osd pool create pg_pool_t::stripe_width behavior

* Check that the default from the configuration options is found in the
  output of osd dump
* Check that specifying an undersized osd_pool_erasure_code_stripe_width
  value is taken into account and padded.

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #1188 from dachary/wip-7339

DNM: define pg_pool_t::stripe_width and set from erasure-code plugin on erasure pool creation

Reviewed-by: Samuel Just <sam.just@inktank.com>