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Re: Ordered Mode vs Journaled Mode



On Tue, 2001-10-02 at 23:59, Andreas Dilger wrote:
> Finally, data journalling is _way_ faster if you are doing synchronous I/O
> like for a mail server.  Both the data and metadata are written in one
> pass to the journal, so no seeking, and you can return control to the
> application knowing the data is safe.  It can then re-order writes to the
> fs safely, since it will be recovered in case of a crash.

Just a comment on this (might be relevant to Mike Fedyk's query too). 
Mail systems do not just have high filesystem integrity requirements
(you can't ack the end of an SMTP transaction until it is committed to
stable storage), but modern systems also have *very* short queue file
lifetimes.   With data journalling, not only do you gain by streaming
the writes into the journal (so normally losing the requirement for some
extra seeks before you can complete the fsync() at the end of the queue
file write), but by the time the kernel threads get to actually updating
the filesystem metadata structures the file has gone again, and probably
been reused... you might never actually get to writing a good proportion
of the queue files to anything other than the journal.

	Nigel.






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