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Date: Sat, 17 Nov 2012 19:50:02 -0700 (MST)
From: Warren Block <wblock@wonkity.com>
To: Gary Aitken <freebsd@dreamchaser.org>
Subject: Re: 9.0 crash, ssd or filesystem problem?
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On Sat, 17 Nov 2012, Gary Aitken wrote:

>> There was a nice Sun white paper by Peter Snyder on tmpfs.  It's linked on the Wikipedia tmpfs page, but Oracle has broken the link.  Google has a rendered version of a PostScript copy (long URL):
>> http://webcache.googleusercontent.com/search?q=cache:EXMeqvhFfrsJ:www.sun3arc.org/papers/OS/tmpfs_virtual_memory_filesystem.ps.gz+tmpfs+white+paper&cd=12&hl=en&ct=clnk&gl=us
>
> Thanks.
> Interesting, I would have thought swap space for something in a tmpfs was
> not allocated until it needed to be swapped out.  As I read it, used tmpfs
> space reserves space in swap.

It uses VM, so tmpfs is a memory disk until you run low on memory, then 
it gets swapped out.  But until then, it's a RAM disk, with the added 
benefit of not eating a fixed chunk of RAM all the time.

Oh, and unless you have a very recent -STABLE or -HEAD, see
http://www.freebsd.org/cgi/query-pr.cgi?pr=bin/168544