QMail and SoftUpdates

[Don Lewis]

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.