Date: Tue, 18 May 2004 16:08:21 -0700 (PDT) From: Don Lewis <truckman@FreeBSD.org> To: julian@elischer.org Cc: mi+mx@aldan.algebra.com Subject: Re: QMail and SoftUpdates Message-ID: <200405182308.i4IN8L7E021763@gw.catspoiler.org> In-Reply-To: <Pine.BSF.4.21.0405180804110.39951-100000@InterJet.elischer.org>
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On 18 May, Julian Elischer wrote: > > > On Mon, 17 May 2004, Don Lewis wrote: > >> On 17 May, Nikita Danilov wrote: >> > Xin LI writes: >> > > On Mon, May 17, 2004 at 01:18:15PM -0400, Mikhail Teterin wrote: >> > > > The link at >> > > > >> > > > http://cr.yp.to/qmail/faq/reliability.html#filesystems >> > > > >> > > > claims, using SoftUpdates for mailqueue is dangerous. Is that still >> > > > true? Thanks! >> > > >> > > Yes, it is dangerous. Same is true for any journalling file systems, >> > > which essentially does the same thing: delayed write of data/metadata. >> > > >> > > Delayed write will make it possible for the Operating System to group >> > > several writes together and write them once, or at least, in a better >> > > order in order to improve performance. However, for the mail case, once >> > > it responds "250", then the remote peer is allowed to remove the message >> > > from its queue. If the system crashes, and the data was not written into >> > > disk, then your message is lost. >> > >> > Unless mail-server did fsync(2) which is guaranteed to return only after >> > data reached stable storage. If file-system doesn't provide such >> > guarantee it's broken, if mail server doesn't call fsync, or >> > fdatasync---it is. Even without any journalling involved. >> >> Based on the information I found using Google, it appears that qmail >> relies on link(2) being synchronous to let it know that a queued message >> is safely on the disk with a known file name before it issues the "250" >> response. I believe this was true without softupdates, but with >> softupdates enabled it is definitely not true. >> > > An fsync will sync ALL directory entries pointing to the file I haven't looked at how qmail works, but my suspicion is that it fsync()s the file and then creates a link (and probably unlinks the old name) to mark the queue file as valid and is not partially written. I think this would work with softupdates if the file were fsync()ed again after the link() call. I won't comment about why this change is unlikely to make it into the code.
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