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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