Re: [uml-devel] Review needed for ubd fixes

[Blaisorblade <blaisorblade@yahoo.it>]

On Wednesday 21 September 2005 20:04, Jeff Dike wrote:
> On Wed, Sep 21, 2005 at 05:49:43PM +0200, Blaisorblade wrote:
[...]
> > Don't think it's reasonable to expect this. I think that filesystems like
> > ext2 will never produce write barriers - they are needed for journaled
> > FS's.

> I thought your point was to rely on the block layer to order overlapping
> writes for us.
The block layer is supposed to merge as far as possible overlapping writes, 
that's reasonable, but not dependable. Say it gets the overlapping request 
*after* it sent the first one - we'll see both. Also, this is trivially true 
for output done through page cache, but for the rest I don't know if explicit 
merging is implemented.

For the buffer cache (i.e. fs metadata) it may still hold, but I'm not sure at 
all. Not that it matters for us - we can't depend on it anyway.

No, I was thinking to you enforcing ordering to comply to what journaled 
filesystems expect.

Btw: even when we aren't using COW, we're supposed to do writes in the order 
the fs passed them to us - at least when write barriers are explicitly sent 
(don't know if there's something to care normally - I know this as a LWN.net 
reader, not as an hacker in this area).

> It sounded reasonable to me :-)

> 				Jeff

-- 
Inform me of my mistakes, so I can keep imitating Homer Simpson's "Doh!".
Paolo Giarrusso, aka Blaisorblade (Skype ID "PaoloGiarrusso", ICQ 215621894)
http://www.user-mode-linux.org/~blaisorblade

	

	
		
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