Date: Mon, 17 May 2004 21:28:17 -0700 (PDT) From: Don Lewis <truckman@FreeBSD.org> To: Nikita@Namesys.COM Cc: mi+mx@aldan.algebra.com Subject: Re: QMail and SoftUpdates Message-ID: <200405180428.i4I4SH7E019389@gw.catspoiler.org> In-Reply-To: <16552.64697.572176.262372@laputa.namesys.com>
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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.
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