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

[Jeff Dike <jdike@addtoit.com>]

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.

				Jeff


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