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Could you elaborate on the semantics of "cache"? I do understand the mirror keyword, but I know next to nothing about ZFS and don't really get what "cache" means, considering bouth your and OPs comment.

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a cache drive is l2arc. Brendan Gregg's post[0] explains it well, but the TLDR is that l2arc is very much like an l2 cache in a cpu. Recent and relevant items are held on a fast drive in case it will be needed in the near future. It is really an L2 cache: L1 is called ARC and is held in dram.

If I recall correctly, the extra indexes and tags mean l2arc requires more memory to work efficiently.

[0]: https://blogs.oracle.com/brendan/entry/test

When your "medium-sized test server" has 128 GB RAM and half a TB worth of SSDs... in 2008! Damn.

You can use a fast disk like an SSD as a 2nd level cache device after ram. So reads will first check the main ram cache first, then the ssd cache disk before finally hitting the underlying storage.

L2ARC is extremely well documented right in the code

https://svnweb.freebsd.org/base/head/sys/cddl/contrib/openso...

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