Messages in this thread | | | Date | Wed, 23 Nov 2005 09:46:56 +0100 | From | Jens Axboe <> | Subject | Re: [PATCH linux-2.6-block:post-2.6.15 08/10] blk: update IDE to use new blk_ordered |
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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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