Clifton Royston wrote:
On Wed, May 25, 2005 at 10:51:58AM -0700, Ken A wrote:
I'm running the latest qpopper on FC3 with user quotas turned on.
With fast-update on, I see "Invalid cross-device link (18)
[pop_updt.c:770]" in the logs. I know this is normal for fast-update
when the spool and temp drop are on different partitions because of the
way rename works.
But, if I put the temp drop on the same filesystem as the mail spool,
users can't check mail when they hit their hard quota. :-(
I'd like to have the temp drop on a different partition so quotas work,
but still use fast-update, since I'd like to keep disk i/o down as much
as possible. I'd appreciate any ideas.
This is inherently impossible due to issues with the Unix quota model
and filesystems. "fast-update" is a simple relink, i.e. rename, to
replace the spool when the ideal case is hit; that is how it can do
essentially no I/O at this step. Obviously this can only happen when
the two files exist on the same filesystem (indeed, in any OS I know
of, not just UNIX.)
However, it's not possible in the UNIX model to have a user quota
that applies only to a portion of a filesystem, or to "all files except
...". That sharply limits the usefulness of hard quotas if you have
the spool and the temp drop on the same filesystem, as you say.
Finally, it would be a severe security risk if qpopper were to hold
superuser privileges until the end of the session, when they would be
needed if one were to juggle file ownership at that point (e.g. by
making the tempdrop file qpopper-owned and group writable until it were
swapped into place with the user's spool.)
AFAIK, nobody's come up with a better way to juggle files that would
reconcile these issues.
-- Clifton
I think found a way to work around this, at least with linux kernel 2.4
and up. The fix is to put the temp drop on a different partition so the
quota system doesn't see it, and then use the vfs feature "mount --bind"
to remount the temp drop filesystem as part of the /var/spool
filesystem. ie: mount --bind /var/tempdrop /var/spool/tempdrop
This allows qpopper to rename files using fast-update, and since the
quota system is looking at the underlying filesystem, it ignores files
in the temp drop directory.
I'm not sure if this is intended behavior for quotas or not, so it may
change if the quota system changes, but it certainly makes the using the
quota system more 'flexible'. Please let me know if you see any problems
with this approach.
Thanks,
Ken Anderson
Pacific.Net
