[ceph-users] Regarding cache tier understanding

Somnath Roy Somnath.Roy at sandisk.com
Thu Aug 7 09:00:51 PDT 2014


Thanks Sage, this helps !

Regards
Somnath

-----Original Message-----
From: Sage Weil [mailto:sweil at redhat.com]
Sent: Thursday, August 07, 2014 7:12 AM
To: Somnath Roy
Cc: ceph-devel at vger.kernel.org; ceph-users at lists.ceph.com; Anirban Ray; Allen Samuels
Subject: Re: Regarding cache tier understanding

On Thu, 7 Aug 2014, Somnath Roy wrote:
> Hi Sage,
>
> I went through the tiering agent code base and here is my
> understanding on the agent behavior. Please let me know if that is correct.
>
>
>
> 1.       Agent will be always in idle state if target_max_bytes or
> target_max_objects not set on the pool irrespective of other tiering
> params set in the pool. dirty_micro and full_micro will not be
> calculated if those two params are zero which is by default I guess.

Yeah

> 2.       Now, flush will be activated if dirty_micro is >
> flush_target. My understanding is, once it is activated it will
> iterate through all the dirty objects and flush all the dirty objects
> which is > cache_min_flush_age. Am I right ?

Yeah

> 3.       For the eviction, if full_micro > 1000000 , the mode set as
> TierAgentState::EVICT_MODE_FULL and all the clean objects are flushed.

Yeah

> 4.       if (full_micro > evict_target), the mode is set as
> TierAgentState::EVICT_MODE_SOME. In this scenario evict_effort is
> calculated and based on hit_set and temp calculation some clean
> objects are evicted. My question is , can we quantify this value ? For
> ex, if the target_full_ratio = 50%, once the eviction is triggered, what %objects will be evicted ?

The effort is calculated based on how far between target_full and 100% we are.  This is mapped onto the estimation of atime.  We generate a histogram of age for the objects we have examined, so after the agent has run a bit we'll have a reasonable idea how the current object's age compares to others and can decide whether this is older or newer than the others; based on that we decide what to do.

> 5.       Also, why we are multiplying with 1000000 always ? Is it
> because in the histogram it is positioned as 0..1000000 ?

Yeah, and we use "micro" units throughout to avoid doing any floating point.

> 6.       I saw the cache_min_evict_age is not been used anywhere, am I
> missing anything ?

It's possible.  The _min_ params are a bit dangerous because they can potentially confuse the cache agent (e.g., what if *all* objects are under the min?  How/when do we decide to ignore the min, or, how/when do we give up trying to find an older object?).

> 7.       The cache_min_flush_age will only be applicable if the flush
> is triggered after crossing the dirty_threshold, right ? If
> dirty_threshold is not breached, the flush age param is never checked.

Right.

> I need to understand the behavior so that we can test the Cache
> tiering properly.
>
> Is there any behavior I am missing here which you want us to test out ?

There is a new behavior just merged into master that controls our decision to promote data into the cache on read; see maybe_handle_cache().  That's not strictly agent behavior per se.  Also, there is now a readforward mode that doesn't promote on read ever, based on our discussion about the performance of flash on read.

Hope this helps!
sage

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