On Sat, 16 Apr 2022, Jens Axboe wrote:
> On 4/16/22 2:05 PM, Eric Wheeler wrote:
> > On Fri, 15 Apr 2022, Christoph Hellwig wrote:
> >> On Fri, Apr 15, 2022 at 10:29:34PM +0800, Ming Lei wrote:
> >>> If ext4 expects the following order, it is ext4's responsibility to
> >>> maintain the order, and block layer may re-order all these IOs at will,
> >>> so do not expect IOs are issued to device in submission order
> >>
> >> Yes, and it has been so since REQ_FLUSH (which later became
> >> REQ_OP_FLUSH) replaced REQ_BARRIER 12 years ago:
> >>
> >> commit 28e7d1845216538303bb95d679d8fd4de50e2f1a
> >> Author: Tejun Heo <[email protected]>
> >> Date: Fri Sep 3 11:56:16 2010 +0200
> >>
> >> block: drop barrier ordering by queue draining
> >>
> >> Filesystems will take all the responsibilities for ordering requests
> >> around commit writes and will only indicate how the commit writes
> >> themselves should be handled by block layers. This patch drops
> >> barrier ordering by queue draining from block layer.
> >
> > Thanks Christoph. I think this answers my original question, too.
> >
> > You may have already answered this implicitly above. If you would be so
> > kind as to confirm my or correct my understanding with a few more
> > questions:
> >
> > 1. Is the only way for a filesystem to know if one IO completed before a
> > second IO to track the first IO's completion and submit the second IO
> > when the first IO's completes (eg a journal commit followed by the
> > subsequent metadata update)? If not, then what block-layer mechanism
> > should be used?
>
> You either need to have a callback or wait on the IO, there's no other
> way.
>
> > 2. Are there any IO ordering flags or mechanisms in the block layer at
> > this point---or---is it simply that all IOs entering the block layer
> > can always be re-ordered before reaching the media?
>
> No, no ordering flags are provided for this kind of use case. Any IO can
> be reordered, hence the only reliable solution is to ensure the previous
> have completed.
Perfect, thanks Jens!
>
> --
> Jens Axboe
>
>
--
Eric Wheeler