Re: [PATCH linux-2.6-block:post-2.6.15 08/10] blk: update IDE to use new blk_ordered

[Jens Axboe <axboe@suse.de>]

On Wed, Nov 23 2005, Bartlomiej Zolnierkiewicz wrote:
> On 11/23/05, Tejun Heo <htejun@gmail.com> wrote:
> > On Tue, Nov 22, 2005 at 09:36:09AM +0100, Bartlomiej Zolnierkiewicz wrote:
> > [--snip--]
> > > >
> > > > Ordered requests are processed in the following order.
> > > >
> > > > 1. barrier bio reaches blk queue
> > > >
> > > > 2. barrier req queued in order
> > > >
> > > > 3. when barrier req reaches the head of the request queue, it gets
> > > >    interpreted into preflush-barrier-postflush requests sequence
> > > >    and queued.  ->prepare_flush_fn is called in this step.
> > > >
> > > > 4. When all three requests complete, the ordered sequence ends.
> > > >
> > > > Adding !drive->wcache test to idedisk_prepare_flush, which in turn
> > > > requires adding ->prepare_flush_fn error handling to blk ordered
> > > > handling, prevents flushes for barrier requests between step#1 and
> > >
> > > Why for !drive->wcache flush can't be consider as successful
> > > like it was before these changes...
> > >
> > > > step#3.  We can still have flush reqeuests between #3 and #4 after
> > > > wcache is turned off.
> > >
> > > ditto
> > >
> >
> > I think we have two alternatives here - both have some problems.
> >
> > 1. make ->prepare_flush_fn return some code to tell blk layer skip
> >    the flush as the original code did.
> >
> > This is what you're proposing, I guess.  The reason why I'm reluctant
> > to take this approach is that there still remains window of error
> > between #3 and #4.  The flush requests could already be prepared and
> > in the queue when ->wcache is turned off.  AFAICS, the original code
> > had the same problem, although the window was narrower.
> >
> > 2. complete flush commands successfully in execute_drive_cmd() if wcache
> >    is not enabled.
> >
> > This approach fixes all cases but the implementation would be a bit
> > hackish.  execute_drive_cmd() will have to look at the command and
> > ide_disk->wcache to determine if a special command should be completed
> > successfully without actually executing it.  Maybe we can add a
> > per-HL-driver callback for that.
> >
> > Bartlomiej, I'm not really sure about both of above approaches.  My
> > humble opinion is that benefits brought by both of above aren't worth
> > the added complexity.  The worst can happen is a few IDE command
> > failures caused by executing flush command on a wcache disabled drive.
> > And that would happen only when the user turns off wcache while
> > barrier sequence is *in progress*.
> >
> > Hmmm... What do you think?  It's your call.  I'll do what you choose.
> 
> Hmm... both solutions sucks.  After second thought I agree with you
> w.r.t to original changes (just remember to document them in the patch
> description).

Me too. Plus on most drives a flush cache command on a drive with write
back caching disabled will succeed, not fail. t13 is about to mandate
that behaviour as well, and honestly I think this is the logical way to
implement it in a drive (the flush just becomes a noop).

So Tejun, where do we stand with this patch series? Any changes over
what you posted last week? I'd like to get this merged in the block
tree, get it in -mm and merged for 2.6.16 if things work out.

-- 
Jens Axboe

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