md devices mounted with async

[RW]

On Sun, 15 Jun 2008 03:08:34 +0200
Kris Kennaway <kris at FreeBSD.org> wrote:

> RW wrote:
> > I meant that a write to the filesystem doesn't require a
> > corresponding write to disk, and the change can stay in memory
> > indefinitely. Presumably, more or less, the same inactive pages get
> > written-out to swap, with or without async.  
> 
> Well, it doesn't necessarily cause a write to disk for each
> filesystem write, but the synchronization mode of the filesystem to
> the backing store is precisely what the async/noasync/sync mount
> options control!

It's not obvious that that's true when the backing-store is swap, I
would have expected that changes would only be written-out when memory
is needed elsewhere rather than to keep the backing-store synchronized.

If I put some big files in /tmp (mounted noasync) the amount of swap
used is often much less the total storage used in /tmp (up to a 1GB
difference, 2/3 of ram), and it can remain like that indefinitely,
which implies that a swap-backed filesystem can remain out of sync with
it's backing-store indefinitely.