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Re: Journal Usage


On Tue, Nov 13, 2001 at 02:52:35PM -0800, Mike Fedyk wrote:

> > > I'd propose something more gradual besides "oops, the journal is full, let's
> > > empty it now..."
> > 
> > The advantage to leaving it as long as possible is that you minimise
> > the amount that needs to be flushed.  If you flush too soon you risk
> > doing quite a bit of unnecessary IO.
> Actually, it looks like this will be needed anyway to deal with large
> ammounts of IO to the journal.  There already seems to be a patch that does
> this out there somewhere a while back (someone mentioned here or on
> ext2-devel).

Will it help?  All you end up doing is increasing the finite amount of
buffering before you stall.  If you consider total fs throughput, it
will _always_ be bottlenecked eventually on the journal.  

Doing journal IO in parallel with the checkpoint will just increase
thrashing, so doing checkpoint earlier just stalls the journal

> Low percentage used (20%<)?  Don't write out if there are transactions writing
> aggressively to the journal. 

To some extent this is beyond our control: we allow the VM to choose
when to write back data which is already journaled.

> One question.  If I write something that goes completely into the journal
> (large 400MB journal, data=journal) the write is still in memory.  If there
> is memory pressure, that'll cause the completed transactions to be written
> out to the FS to free some memory...  Does that mean that I need 400MB of
> memory that stays cached to use my entire 400MB journal?



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