From owner-freebsd-fs@FreeBSD.ORG  Wed Jun 22 04:51:56 2011
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Date: Wed, 22 Jun 2011 06:51:53 +0200
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From: Wiktor Niesiobedzki <bsd@vink.pl>
To: Artem Belevich <art@freebsd.org>
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Cc: freebsd-fs@freebsd.org
Subject: Re: ZFS L2ARC hit ratio
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2011/6/22 Artem Belevich <art@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...

Cheers,

Wiktor