Re: vm/fs meetup details

["Jörn Engel" <joern@logfs.org>]

On Fri, 6 July 2007 09:52:14 -0400, Chris Mason wrote:
> On Fri, 6 Jul 2007 23:42:01 +1000 David Chinner <dgc@sgi.com> wrote:
> 
> > 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.

Interesting idea.  Is it possible to attach several address spaces to an
inode?  That would cure some headaches.

> At least in the case of btrfs, the perfect order for sync is disk
> order ;)  COW happens when blocks are changed for the first time in a
> transaction, not when they are written out to disk.  If logfs is
> writing things out some form of tree order, you're going to have to
> group disk allocations such that tree order reflects disk order somehow.

I don't understand half of what you're writing.  Maybe we should do
another design session on irc?

At any rate, logfs simply writes out blocks.  When it is handed a page
to write, the corresponding block is written.  Allocation happens at
writeout time, not earlier.  Each written block causes a higher-level
block to get changed, so that is written immediatly as well, until the
next higher level is the inode.

I would like to instead just dirty the higher-level block, so that
multiple changes can accumulate before indirect blocks are written.  And
I have no idea how transactions relate to all this.

> But, the part where we toss leaves first is definitely useful.

Shouldn't LRU ordering already do that.  I can even imagine cases when
leaves should be tossed last and LRU ordering would dtrt.

Jörn

-- 
The competent programmer is fully aware of the strictly limited size of
his own skull; therefore he approaches the programming task in full
humility, and among other things he avoids clever tricks like the plague.
-- Edsger W. Dijkstra
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