From owner-freebsd-questions@FreeBSD.ORG Sun Jun 15 02:55:59 2008 Return-Path: Delivered-To: freebsd-questions@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34]) by hub.freebsd.org (Postfix) with ESMTP id 32A421065676 for ; Sun, 15 Jun 2008 02:55:59 +0000 (UTC) (envelope-from fbsd06+W5=e1c463e1@mlists.homeunix.com) Received: from mxout-03.mxes.net (mxout-03.mxes.net [216.86.168.178]) by mx1.freebsd.org (Postfix) with ESMTP id 0488F8FC0C for ; Sun, 15 Jun 2008 02:55:58 +0000 (UTC) (envelope-from fbsd06+W5=e1c463e1@mlists.homeunix.com) Received: from gumby.homeunix.com. (unknown [87.81.140.128]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.mxes.net (Postfix) with ESMTP id B252523E49A for ; Sat, 14 Jun 2008 22:55:57 -0400 (EDT) Date: Sun, 15 Jun 2008 03:55:55 +0100 From: RW To: freebsd-questions@freebsd.org 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> X-Mailer: Claws Mail 3.4.0 (GTK+ 2.12.10; i386-portbld-freebsd7.0) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Subject: Re: md devices mounted with async X-BeenThere: freebsd-questions@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: User questions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sun, 15 Jun 2008 02:55:59 -0000 On Sun, 15 Jun 2008 03:08:34 +0200 Kris Kennaway 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.