[Gllug] re: qmail file-system syncing

[t.clarke]

Note: I did enclose weakness in quotes !

I accept that syncing disc-writes is the most reliable way of ensuring emails
are not lost, but my understanding is that qmail actually syncs the whole
file-system every time rather than simply the contents of the file being
written.

Bearing in mind the availability of journalling filesystems, the reliability
of modern hardware, the cheapness of UPS's, I simply wonder if the sledgehammer
sync approach is really necessary in SOME scenarios.    OK, I maybe tempting
providence here, but we have never had to call upon the safety of file-system
syncing as yet, 'cos the O-S hasn't crashed and the machine is on a UPS.
The most likely cause of loss would seem to be a disc crash - and syncing
probably wont help there !

I gather that certain linux filesystems now support syncing of metadata, so
that it should be possible to sync a file-write to disc together with updates
to corresponding inodes, directories etc, without having to sync the whole
flaming filesystem.  Of course, I may have misunderstood what qmail does in
this respect.  If qmail does sync the whole filesystem, it would clearly pay
to give it its 'own' filesystem  - something that wasn't made clear to me when
we installed it !


Tim

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