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); Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx
