Date: Mon, 25 May 1998 17:39:51 -0500 (EST) From: "John S. Dyson" <toor@dyson.iquest.net> To: robert@cyrus.watson.org (Robert Watson) Cc: dag-erli@ifi.uio.no, stable@FreeBSD.ORG Subject: Re: vnconfig + swap with sparse files Message-ID: <199805252239.RAA10934@dyson.iquest.net> In-Reply-To: <Pine.BSF.3.96.980525170957.5875A-100000@cyrus.watson.org> from Robert Watson at "May 25, 98 05:15:57 pm"
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> > On 25 May 1998, Dag-Erling Coidan [iso-8859-1] Smørgrav wrote: > > > Robert Watson <robert@cyrus.watson.org> writes: > > > I rebooted to stop swapping on the file, and modified my creation program > > > to drop a \0 at the beginning of each block. Sure enough, starting > > > swapping this time worked fine. > > > > What you should have done in the first place was: > > > > # dd if=/dev/zero of=swapfile bs=1048576 count=128 > > > > A sparse file cannot possibly work as swap since it does not have any > > space allocate to it on the disk, and you *want* to allocate space on > > the disk - where else are you going to swap out your data? > > I would argue that more intuitive behavior would be for the pager to get > back 0'd pages, as all other users of sparse files do. When I mmap my > sparse file, I get 0's :). While this is more intuitive, it may not in > reality be feasible. However, a note in vn(4) under BUGS indicating that > the vn driver does not automatically allocate pages for sparse files when > used, or documenting its error return on attempting to read or write an > unallocated block in a sparse file would make sense. > > Indeed, I did not expect the pager to swap to my sparse file -- I expected > the file to become un-sparse as it went along. > It is a problematical issue to allocate space during swap pager pageouts. In order to minimize the deadlock issues, the file has to be pre-allocated. When I get swapoff working, I will also make swapon check for files to be properly preallocated. John To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-stable" in the body of the message
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