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Comment by hoistor | Hacker News Reader

I don't think the author of this post really understands ZFS.

Is that a 4 disk mirror? You'd be much faster using a raid10 there.

ZFS 'cache' devices are't for write cache either, you'd want a log device for write perf reasons if that's your constraint... Even then it's unlikely to make a difference on a workstation workload with 16GB of memory..

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I think he might just be glossing over some details? At any rate, from what I understand ZFS has a 'zil' write cache for small log writes, and a l2arc for common reads that don't fit into memory. This is just a dev setup, but for real work I think you'd want your zil to also be mirrored so you don't lose writes if an ssd goes bad. For anyone interested this link has a few more details: http://www.45drives.com/wiki/index.php/FreeNAS_-_What_is_ZIL...

Yeah.. it is a subtle difference between

  zpool create home mirror a b c d cache e

and

  zpool create home mirror a b mirror c d cache e

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.

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...

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.

Correct me if Im wrong, but he just built a pool with 3 layers of parity right? The first drive with three drives mirroring it? Where as the later is effectively RAID10, a stripe across 2 mirrored vdevs.

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