Re: Cache tier READ_FORWARD transition

[Mark Nelson <mark.nelson@inktank.com>]

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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