keyvaluestore backend metadata overhead

[Chris Pacejo <cpacejo@xxxxxxxxxxxxxxxx>]

Hi, we've been experimenting with the keyvaluestore backend, and have found that, on every object write (e.g. with `rados put`), a single transaction is issued containing an additional 9 KeyValueDB writes, beyond those which constitute the object data.  Given the key names, these are clearly all metadata of some sort, but this poses a problem when the objects themselves are very small.  Given the default strip block size of 4 KiB, with objects of size 36 KiB or less, half or more of all key-value store writes are metadata writes.  With objects of size 4 KiB or less, the metadata overhead grows to 90%+.

Is there any way to reduce the number of metadata rows which must be written with each object?

(Alternatively, if there is a way to convince the OSD to issue multiple concurrent write transactions, that would also help.  But even with "keyvaluestore op threads" set as high as 64, and `rados bench` issuing 64 concurrent writes, we never see more than a single active write transaction on the (multithread-capable) backend.  Is there some other option we're missing?)

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