On Sun, Jan 27, 2008 at 01:47:18PM -0800, kelsey hudson wrote:
David Brown wrote:
I run the cyrus imap server as my mail server. One of the things it
maintains is a database is visited message ids, to avoid duplicate message
delivery.
This database is maintained with Berkeley DB, configured to write a
logfile
and several __db.001 through __db.006 files.
The problem I'm having is that I can't back these files up reliably.
What are you using to do your backups?
It doesn't matter what I use. If the mtime/ctime of the file doesn't
change, no backup program is going to realize that it needs to back them up
again.
Right now I'm using GNU tar, because it is the _only_ backup software I can
find that understands how to do incremental backups correctly.
- Cyrus somehow seems to be able to change these files without changing
the ctime or mtime. Without the change, my backup doesn't think
they've changed.
- Even on exit, they don't get touched. A restart of imapd does touch
the files.
I'm wondering if I should worry about it. Perhaps the log file contains
enough information to reconstruct whatever state the db files should be in
when they were last touched. It also might not matter, the worst case
being a few days of duplicate messages being delivered.
What I'm really wondering is how the program can be modifying the files
without touching the timestamps. I tried using mmap myself, and I can't
change the data without updating the mtime.
MMIO and O_DIRECT should allow this.
I'll have to study that path through the kernel. Even with O_DIRECT, page
cache flushes should still be going through the file interface. I'm not
sure Linus is real interested in fixing it, since he feels that O_DIRECT
needs to go away, and databases should be using madvise.
Right now, I suspend the mail server, touch the database files, back it up,
and then restart mail/imap.
David
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