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 <tj@xxxxxxxxxx> > >> 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
