On Sun, Mar 03, 2019 at 10:37:59AM +0800, Bob Liu wrote: > On 3/1/19 5:49 AM, Dave Chinner wrote: > > On Thu, Feb 28, 2019 at 10:22:02PM +0800, Bob Liu wrote: > >> On 2/19/19 5:31 AM, Dave Chinner wrote: > >>> On Wed, Feb 13, 2019 at 05:50:35PM +0800, Bob Liu wrote: > >>>> Motivation: > >>>> When fs data/metadata checksum mismatch, lower block devices may have other > >>>> correct copies. e.g. If XFS successfully reads a metadata buffer off a raid1 but > >>>> decides that the metadata is garbage, today it will shut down the entire > >>>> filesystem without trying any of the other mirrors. This is a severe > >>>> loss of service, and we propose these patches to have XFS try harder to > >>>> avoid failure. > >>>> > >>>> This patch prototype this mirror retry idea by: > >>>> * Adding @nr_mirrors to struct request_queue which is similar as > >>>> blk_queue_nonrot(), filesystem can grab device request queue and check max > >>>> mirrors this block device has. > >>>> Helper functions were also added to get/set the nr_mirrors. > >>>> > >>>> * Introducing bi_rd_hint just like bi_write_hint, but bi_rd_hint is a long bitmap > >>>> in order to support stacked layer case. > >>>> > >>>> * Modify md/raid1 to support this retry feature. > >>>> > >>>> * Adapter xfs to use this feature. > >>>> If the read verify fails, we loop over the available mirrors and retry the read. > >>> > >>> Why does the filesystem have to iterate every single posible > >>> combination of devices that are underneath it? > >>> > >>> Wouldn't it be much simpler to be able to attach a verifier > >>> function to the bio, and have each layer that gets called iterate > >>> over all it's copies internally until the verfier function passes > >>> or all copies are exhausted? > >>> > >>> This works for stacked mirrors - it can pass the higher layer > >>> verifier down as far as necessary. It can work for RAID5/6, too, by > >>> having that layer supply it's own verifier for reads that verifies > >>> parity and can reconstruct of failure, then when it's reconstructed > >>> a valid stripe it can run the verifier that was supplied to it from > >>> above, etc. > >>> > >>> i.e. I dont see why only filesystems should drive retries or have to > >>> be aware of the underlying storage stacking. ISTM that each > >>> layer of the storage stack should be able to verify what has been > >>> returned to it is valid independently of the higher layer > >>> requirements. The only difference from a caller point of view should > >>> be submit_bio(bio); vs submit_bio_verify(bio, verifier_cb_func); > >>> > >> > >> We already have bio->bi_end_io(), how about do the verification inside bi_end_io()? > >> > >> Then the whole sequence would like: > >> bio_endio() > >> > 1.bio->bi_end_io() > >> > xfs_buf_bio_end_io() > >> > verify, set bio->bi_status = "please retry" if verify fail > >> > >> > 2.if found bio->bi_status = retry > >> > 3.resubmit bio > > > > As I mentioned to Darrick, this isn't cwas simple as it seems > > because what XFS actually does is this: > > > > IO completion thread Workqueue Thread > > bio_endio(bio) > > bio->bi_end_io(bio) > > xfs_buf_bio_end_io(bio) > > bp->b_error = bio->bi_status > > xfs_buf_ioend_async(bp) > > queue_work(bp->b_ioend_wq, bp) > > bio_put(bio) > > <io completion done> > > ..... > > xfs_buf_ioend(bp) > > bp->b_ops->read_verify() > > ..... > > > > IOWs, XFS does not do read verification inside the bio completion > > context, but instead defers it to an external workqueue so it does > > not delay processing incoming bio IO completions. Hence there is no > > way to get the verification status back to the bio completion (the > > bio has already been freed!) to resubmit from there. > > > > This is one of the reasons I suggested a verifier be added to the > > submission, so the bio itself is wholly responsible for running it, > > But then completion time of an i/o would be longer if calling verifier function inside bio_endio(). > Would that be a problem? No, because then we don't have to do it in the filesystem. i.e. the filesystem doesn't complete the IO until after the verifier has run, so it from the perspective of the waiting reader is doesn't matter where it is run because the overall I/O latency is the same. Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx
