Re: strange cache tier behaviour with cephfs

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Hello,

On Tue, 14 Jun 2016 01:57:49 +0200 Oliver Dzombic wrote:

> Hi,
> 
> yeah, for sure you are right !
> 
> Sorry...
> 
> Its Centos 7 ( default kernel 3.10 )
> ceph version 10.2.1 (3a66dd4f30852819c1bdaa8ec23c795d4ad77269)
> 
> ----
> 
> In the very end i want/need a setup where all is going to the cache tier
> ( read and write ).
> 
> Writes shall be, after some time without modification, written to the
> cold pool.
>
That's not how cache tiering works (by default). 
See * below.
 
> Read shall be, as long as they are frequently red, remain on the cache
> pool.
> 
> To me, writeback mode should do that. But somehow does not.
> 
> Since my thread opening mail 1,5 houres has been passed by.
> 
> And its still not evicted. So i still have this objects inside the cache
> pool, while after 3600 seconds they should be gone.
> 
> So there is some major problem here, as it seems to me.
> 
No there isn't. 
You did read my "Cache tier operation clarifications" thread a few months
back, time to re-read it.

Flushes (writes to the cold pool) happen when the dirty ratios are
reached, never before that. 

Evictions are just zero'ing clean (previously flushed) objects when the
full ratio is reached.

* 
You might be able to achieve that by setting cache_target_dirty_ratio to
0, but I don't see the advantage in having objects flushed half an hour
later by default. 
If your cache pool fails, you'll still be stuck with tons of broken PGs, a
dead pool for all intents and purposes.
While running the cache tier "normally" may allow you to do flushes during
off-peak hours if you cache pool is large enough.

Christian
-- 
Christian Balzer        Network/Systems Engineer                
chibi@xxxxxxx   	Global OnLine Japan/Rakuten Communications
http://www.gol.com/
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