Date: Mon, 17 May 2004 21:56:09 +0400 From: Nikita Danilov <Nikita@Namesys.COM> To: Xin LI <delphij@frontfree.net> Cc: Mikhail Teterin <mi+mx@aldan.algebra.com> Subject: Re: QMail and SoftUpdates Message-ID: <16552.64697.572176.262372@laputa.namesys.com> In-Reply-To: <20040517174836.GA983@frontfree.net> References: <200405171318.15200@misha-mx.virtual-estates.net> <20040517174836.GA983@frontfree.net>
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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. Nikita.
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