Re: vm/fs meetup details

[David Chinner <dgc@sgi.com>]

On Fri, Jul 06, 2007 at 12:26:23PM +0200, Jörn Engel wrote:
> On Fri, 6 July 2007 20:01:10 +1000, David Chinner wrote:
> > On Fri, Jul 06, 2007 at 04:26:51AM +0200, Nick Piggin wrote:
> > > 
> > > Keep in mind that the way to get the most out of this meeting is for the
> > > fs people to have topics of the form "we'd really like to do X, can we
> > > get some help from the VM"? Or vice versa from vm people.
> > 
> > *nod*
> > 
> > But, surprisingly enough, the above work is relevent to this forum because
> > of two things:
> > 
> > 	- we've had to move to direct I/O and user space caching to work
> > 	around deficiencies in kernel block device caching under memory
> > 	pressure....
> > 
> > 	- we've exploited techniques that XFS supports but the VM does not.
> > 	i.e. priority tagging of cached metadata so that less important
> > 	metadata is tossed first (e.g. toss tree leaves before nodes and nodes
> > 	before roots) when under memory pressure.
> 
> And the latter is exactly what logfs needs as well.  You certainly have me
> interested.
> 
> I believe it applies to btrfs and any other cow-fs as well.  The point is
> that higher levels get dirtied by writing lower layers.  So perfect
> behaviour for sync is to write leaves first, then nodes, then the root.  Any
> other order will either cause sync not to sync or cause unnecessary writes
> and cost performance.

Hmmm - I guess you could use it for writeback ordering. I hadn't
really thought about that. Doesn't seem a particularly efficient way
of doing it, though. Why not just use multiple address spaces for
this? i.e. one per level and flush in ascending order.

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group
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