[ ... ]
Actually, the thing I use swap for most now is to make sure I can allocate
large temporary file systems without consuming excessive kernel address
space. I.e., I'll often create a 512mb swap-backed md device for /tmp,
and make sure I have enough swap to fully back it and everything else,
even though the chances are I won't touch it in normal operation. I just
don't want to run out in the event something does need it...
I first saw this implemented under Solaris via the "tmpfs" filesystem mount type, and I agree with you that it tends to function quite well. By the time Solaris transitioned from 2.6 to 2.7 (aka Solaris 7), having /tmp be memory-based was the default system configuration, and I would like to see FreeBSD pursue the same course.
I seem to recall that Solaris sets the paging priority of processes higher than tmpfs so that the system will write out of the contents of the memory-based filesystem rather than reducing process working sets if the system encounters memory pressure. I'm not sure to what extent this is applicable to FreeBSD's md device.
Also, will FreeBSD double-buffer md based devices if it does need to scribble their contents to disk, or is something like what Mach calls a "unified buffer cache" available?
-- -Chuck
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