Re: vm/fs meetup details

["Jörn Engel" <joern@logfs.org>]

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.

Jörn

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