QMail and SoftUpdates
julian at elischer.org
Tue May 18 08:05:48 PDT 2004
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
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