[Dovecot] Dovecot "sleep" for short snapshots

[Peer Heinlein]

Making consistent backups from Dovecot with mdbox requires a snapshoted
filesystem with (at least) a short Downtime for Dovecot.

I'd like to avoid killing several thenthousand IMAP-sessions for 60
seconds. Users don't like that... And the admin dislakes that too...

What about a "short sleep" mechanism for Dovecot, where all writes are
finished and new writes are easily delayed?

I'd like to following process:

*) "doveadm sleep" puts Dovecot in sleep-Mode:
  -- All existing writes are finished (before "doveadm sleep" is allowed
 to exit)
  -- All new writes are simply delayed
  -- All readonly-operations could work normally, so users can fetch and
read mails

*) The the admin can create a LVM-/SAN-snapshot of the filesystems

*) "doveadm unsleep" will finish the magic

Overall the whole process will take just some seconds.

Users will recognize a simple write-delay, but they can work normally
and the aren't unconnected at all. If the snapshot is really fast the
users won't recognize the whole situation at all.


Peer

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