QMail and SoftUpdates

[Julian Elischer]

On Tue, 18 May 2004, Xin LI wrote:

> 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.
> 
> Mounting your mailqueue slice with -noatime is a good idea, too.
> 
> Please note that disabling SoftUpdates will drastically decrease write
> performance on that slice. Of course, if your system often encounter
> crash or power failure, it will be a good idea to disable it.

Softupdates is NOT unsafe for this task IF the app does an fsync()
before acknowldging the incoming mail. Fsync under Soft updates will not
return until all data and metadata associated with the file
has been written to disk. This will make the write as slow as a sync 
munted filesystem, but all other writes on the disk will proceeed at 
soft-updates speed.



> 
> Cheers,
> -- 
> Xin LI <delphij frontfree net>	http://www.delphij.net/
> See complete headers for GPG key and other information.
> 
>