[Dovecot] How to get rid of locks

[Timo Sirainen]

On Sat, 2007-04-07 at 22:30 +0300, Timo Sirainen wrote:
> I just figured out that O_APPEND is pretty great. If the operating  
> system updates seek position after writing to a file opened with  
> O_APPEND, writes to Dovecot's transaction log file can be made  
> lockless.

Well, almost. Log rotation isn't possible without some sort of locking.
But the locks could still be reduced:

 - Normally keep the .log file read-locked all the time (multiple
processes can have it read-locked)
 - Write to it with O_APPEND
 - If you notice that the log is going to be rotated soon, drop the read
lock and acquire it only for the duration of appends
 - When the log is wanted to be rotated, try to get a write-lock. If it
fails, try again later. If it succeeds, it's safe to rotate the log.

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