ZFS L2ARC hit ratio

Wiktor Niesiobedzki bsd at vink.pl
Wed Jun 22 04:51:56 UTC 2011

2011/6/22 Artem Belevich <art at freebsd.org>:
> L2ARC is filled with items evicted from ARC. The catch is that L2ARC
> writes are intentionally throttled. When L2ARC is empty writes happen
> at a higher rate, but it's still intentionally low so that
> read-optimized cache device does not wear out too soon. The bottom
> line is that not all the data spilled out of ARC ends up in L2ARC on
> the first try. Re-run your experiment again and you would probably see
> some improvement in L2ARC hit rates.

I've run the experiment 3 times with no extent. Funny thing is:
- in first run, I see a lot of write activity against cache device
- in second run, I see no write activity against cache device, nor read activity

So my guess is, that anyhow, ZFS cache layer knows, that this file is
*there*, though it decides not to serve it from L2ARC...



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