journal file size limit

Mark Andrews Mark_Andrews at isc.org
Mon Oct 13 21:46:39 UTC 2008


In message <Prayer.1.3.0.0810131719190.19248 at hermes-1.csi.cam.ac.uk>, Chris Tho
mpson writes:
> On Oct 13 2008, Jack Tavares wrote:
> 
> >I see the options clause to limit the journal file size, but I was wondering
> >what happens when the limit is reached?
> > 
> >Is everything in the file (after ensuring that it has been persisted to the 
> >zone file) wiped out and does the file start over?
> > 
> >are N records removed to make room? (with N being some either fixed or 
> >variable number based on the max size of the file specified)?
> > 
> >ie:
> >file size 1M.
> >1M size gets reached
> > 
> >Does the file get emptied/deleted and start over
> > 
> >Does the file get reduced by 10%, 50% ?
> 
> Roughly the last half of the journal file is retained. (Always subject
> to the constraint that journal entries for changes not yet written to
> the zone file are never discarded.)
> 
> There used to be a sneaky way of doing this in place: the index was
> rewritten to invalidate the entries in the first half, the remaining
> entries were copied to the space thus freed, and then the index was
> updated to point to the copy. (The index is 512 bytes at the start
> of the file, assumed to be written atomically.) But I think that
> BIND 9.4.x (and later?) no longer does this, but always writes a new 
> file and renames: maybe there were too many corner cases?

	Replay from the journal and truncation couldn't occur
	simultaneously.  It ended up being simpler to re-write and
	let the replay continue from the now unlinked journal on
	filesystems that support this.  For filesystems that don't
	support this we do other things.

	Mark
-- 
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: Mark_Andrews at isc.org


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