Date: Sun, 15 Jun 2008 03:55:55 +0100 From: RW <fbsd06@mlists.homeunix.com> To: freebsd-questions@freebsd.org Subject: Re: md devices mounted with async Message-ID: <20080615035555.0b5d4b1c@gumby.homeunix.com.> In-Reply-To: <48546B92.5050906@FreeBSD.org> References: <20080614224742.17316919@gumby.homeunix.com.> <48545212.4040006@FreeBSD.org> <20080615013158.7dd19cf0@gumby.homeunix.com.> <48546B92.5050906@FreeBSD.org>
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On Sun, 15 Jun 2008 03:08:34 +0200 Kris Kennaway <kris@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.
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