Michael, first, I wanted to thank you for doing this work, and
especially for publishing the results. I'd love to see more such work.
I have some questions about the NFS server used that I didn't see
answered in the paper.
First, what kernel was used in Debian? I've been using XFS with 2.4.20
(for something other than our mail store server) for some time, and
despite claims to the contrary, its performance is in many ways really
quite poor. In particular, it has a number of problems that make it
especially bad for Maildir. Initial scans of directories tend to be
very slow. This would create a long delay after logging in to a POP3
server, before getting a message list. File system operations also tend
to be extremely slow. Deleting files is a very expensive operation, and
since that is the normal course of action on a POP3 store (though I
didn't get the impression that your clients were doing that), this
further degrades the performance of the system. Updating the access
time on files will normally be done when retrieving mail, as well, which
adds another very expensive filesystem operation to the transaction.
Which leads me to the other question: What mount options did you use
for the XFS volume? I've found that the "noatime" option is essential
on many access patterns, especially when you have large directories of
small files which will be scanned. Using the "noatime" option
substantially reduces the amount of disk write activity that is
required, which normally will seriously degrade read performance.
If you're willing to share, and still have the materials, I'd like to
take some of your scripts and follow up on some of those details to see
how performance characteristics change on filesystem based use.
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