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From Mark Harrison <m...@omniti.com>
Subject Re: Cache almost full. What next ?
Date Tue, 04 Feb 2014 19:50:22 GMT
That document clears a few things up - If I understand correctly now,
evacuation is the mechanism used to prevent an object from being
overwritten/evicted
(https://trafficserver.readthedocs.org/en/latest/arch/cache/cache-arch.en.html#evacuation-mechanics
and the following section). Does that mean that TS-1508 is still valid
or is it that if I'm seeing 0 for that stat that nothing is being
evacuated?

I wonder also if it might be possible to just have whatever process
writes to the cache simply skip over any items marked for evacuation
or that are pinned, which would eliminate the copy.

Thanks for the info on counting the cache wraps, that makes sense and
looks like it would give a decent estimate, which is what I'm looking
for.

On Tue, Feb 4, 2014 at 1:20 PM, Alan M. Carroll
<amc@network-geographics.com> wrote:
> For anyone who doesn't mind getting their brain dirty, there is some documentation on
how the cache works here[1]. A key point to keep in mind is that the TS cache is a circular
buffer, so "full" is a rather vague concept for it. That is, non-stale content can be evicted
even if the cache is not technically full.
>
> [1] https://trafficserver.readthedocs.org/en/latest/arch/cache/cache.en.html
>
> Tuesday, February 4, 2014, 10:30:36 AM, you wrote:
>
>> A related question I have is what would be the best way to determine
>> if your cache misses are a consequence of otherwise fresh content
>> being evicted from the cache?
>
> That's quite difficult currently. My clients are not overly pleased with that situation
either and I am currently working for them on improving the cache API to make this much easier.
Hopefully I will be able to present some progress on this at ApacheCon/TS Summit.
>
> Because of the way the cache works, it's not really feasible for it to actually know
if it is evicting non-stale content. One thing you could do is track cache write bytes and
compare that to the total cache size, which should give you a rough estimate of how often
the cache wraps. It might be reasonable to put in a specific statistic that counts such wraps.
>
> It would be possible to detect an eviction on a cache miss, somewhat reliably. I don't
know if that is tracked specifically though.
>



-- 
Mark Harrison
Lead Site Reliability Engineer
OmniTI

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