caching get_chunk_size()

[Loic Dachary <loic@dachary.org>]

[-- Attachment #1: Type: text/plain, Size: 1515 bytes --]

[cc' ceph-devel]

Hi Sam,

Assuming https://github.com/ceph/ceph/pull/1144 gets merged, MONs can query the plugin for a given pool or crush ruleset conveniently:

https://github.com/dachary/ceph/blob/15c90fbf61445063967fc80ee1c916d70c593a54/src/mon/OSDMonitor.cc#L2803

      ErasureCodeInterfaceRef erasure_code;
      err = get_erasure_code(properties, &erasure_code, ss);
      if (err == 0)
         *size = erasure_code->get_chunk_count();

And OSDs can do something similar (and must whenever an erasure coded pool is instantiated). If I understood correctly, you would like the result of erasure_code->get_chunk_size(some_osd_config_opt_value_defining_the_stripe_size) to be set as a new value in pg_pool_t https://github.com/dachary/ceph/blob/15c90fbf61445063967fc80ee1c916d70c593a54/src/osd/osd_types.h#L700 when the pool is created. 

The benefit of having pg_pool_t::chunk_size would be 

a) the value is shared cluster wide and does not depend on the specific version of the OSD/MON
b) if the value is needed frequently in a context where the plugin would have to be loaded, this will improve performances

However, whenever an OSD instantiates a PG that belongs to a pool, it must assert that the get_chunk_count() returned by the plugin matches the value stored in pg_pool_t::chunk_size otherwise it will encode inconsistently. 

If my reasoning matches yours, I'll implement the change. Otherwise, please correct me :-)

Cheers

-- 
Loïc Dachary, Artisan Logiciel Libre


[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 263 bytes --]