On 07/07/2014 02:29 PM, Sage Weil wrote:
On Mon, 7 Jul 2014, Luis Pabon wrote:
Hi all,
I am working on OSDMonitor.cc:5325 and wanted to confirm the following
read_forward cache tier transition:
readforward -> forward || writeback || (any && num_objects_dirty == 0)
forward -> writeback || readforward || (any && num_objects_dirty == 0)
writeback -> readforward || forward
Is this the correct cache tier state transition?
That looks right to me.
By the way, I had a thought after we spoke that we probably want something
that is somewhere inbetween the current writeback behavior (promote on
first read) and the read_forward behavior (never promote on read). I
suspect a good all-around policy is something like promote on second read?
This should probably be rolled into the writeback mode as a tunable...
That would be a good start I think. What about some kind of scheme that
also favours promoting small objects over larger ones? It could be as
simple as increasing the number of reads necessary to do a promotion
based on the object size.
ie something like:
<= 64k object = 1 read
<= 512k object = 2 read
else 3 read
That would make the behaviour for default RBD object sizes always 3
read, but could keep big objects out of the cache tier for RGW.
Mark
sage
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