On 2019/3/26 9:21 下午, Thorsten Knabe wrote: > Hello, > > there seems to be a serious problem, when running bcache on top of a > degraded RAID-6 (MD) array. The bcache device /dev/bcache0 disappears > after a few I/O operations on the affected device and the kernel log > gets filled with the following log message: > > bcache: bch_count_backing_io_errors() md0: IO error on backing device, > unrecoverable > It seems I/O request onto backing device failed. If the md raid6 device is the backing device, does it go into read-only mode after degrade ? > Setup: > Linux kernel: 5.1-rc2, 5.0.4, 4.19.0-0.bpo.2-amd64 (Debian backports) > all affected > bcache backing device: EXT4 filesystem -> /dev/bcache0 -> /dev/md0 -> > /dev/sd[bcde]1 > bcache cache device: /dev/sdf1 > cache mode: writethrough, none and cache device detached are all > affected, writeback and writearound has not been tested > KVM for testing, first observed on real hardware (failing RAID device) > > As long as the RAID6 is healthy, bcache works as expected. Once the > RAID6 gets degraded, for example by removing a drive from the array > (mdadm --fail /dev/md0 /dev/sde1, mdadm --remove /dev/md0 /dev/sde1), > the above-mentioned log messages appear in the kernel log and the bcache > device /dev/bcache0 disappears shortly afterwards logging: > > bcache: bch_cached_dev_error() stop bcache0: too many IO errors on > backing device md0 > > to the kernel log. > > Increasing /sys/block/bcache0/bcache/io_error_limit to a very high value > (1073741824) the bcache device /dev/bcache0 remains usable without any > noticeable filesystem corruptions. If the backing device goes into read-only mode, bcache will take this backing device as a failure status. The behavior is to stop the bcache device of the failed backing device, to notify upper layer something goes wrong. In writethough and writeback mode, bcache requires the backing device to be writable. Thanks. -- Coly Li
