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

[Blaisorblade <blaisorblade@yahoo.it>]

On Wednesday 21 September 2005 22:45, Jeff Dike wrote:
> On Wed, Sep 21, 2005 at 09:06:38PM +0200, Blaisorblade wrote:
> > 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.
>
> Yeah, I'm concerned about O_DIRECT.  Also, what happens when we write a
> page to disk, and immediately afterwards, it gets dirty again and written
> again? Is there a barrier between the two?
>
> > No, I was thinking to you enforcing ordering to comply to what journaled
> > filesystems expect.
>
> Yeah, but there's more ordering than that.
>
> > 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).
>
> We get requests from the block layer, not the fs.  And physical disks are
> allowed to reorder requests, so I think there is some flexibility there.
I expect the FS to send the barriers - they don't make sense otherwise.

In fact, grep -i barrier fs/jbd/*.c gives some results, which point to 
BH_Ordered.

And after grepping it in reiserfs, I found that fs/buffer.c:submit_bh() cares 
about it (i.e. looks at the buffer head and sees if it is ordered), and sends 
it down to the elevator.
-- 
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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