Re: Performance improvement suggestion

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> 1. Write object A from client.
> 2. Fsync to primary device completes.
> 3. Ack to client.
> 4. Writes sent to replicas.

As mentioned in the discussion this proposal is the opposite of
what the current policy, is, which is to wait for all replicas
to be written before writes are acknowledged to the client:

   "After identifying the target placement group, the client
   writes the object to the identified placement group's primary
   OSD. The primary OSD then [...] confirms that the object was
   stored successfully in the secondary and tertiary OSDs, and
   reports to the client that the object was stored

A more revolutionary option would be for 'librados' to write in
parallel to all the "active set" OSDs and report this to the
primary, but that would greatly increase client-Ceph traffic,
while the current logic increases traffic only among OSDs.

> So I think that to maintain any semblance of reliability,
> you'd need to at least wait for a commit ack from the first
> replica (i.e. min_size=2).

Perhaps it could be similar to 'k'+'m' for EC, that is 'k'
synchronous (write completes to the client only when all at
least 'k' replicas, including primary, have been committed) and
'm' asynchronous, instead of 'k' being just 1 or 2.
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