On Mon, 16 May 2016, Tim Small wrote: > On 08/05/16 19:39, James Johnston wrote: > > I've run into a problem where the bcache writeback cache can't be flushed to > > disk when the backing device is a LUKS / dm-crypt device and the cache set > > has > > a non-default bucket size. Basically, only a few megabytes will be flushed > > to > > disk, and then it gets stuck. Stuck means that the bcache writeback task > > thrashes the disk by constantly reading hundreds of MB/second from the > > cache set > > in an infinite loop, while not actually progressing (dirty_data never > > decreases > > beyond a certain point). > > > [...] > > > The situation is basically unrecoverable as far as I can tell: if you > > attempt > > to detach the cache set then the cache set disk gets thrashed extra-hard > > forever, and it's impossible to actually get the cache set detached. The > > only > > solution seems to be to back up the data and destroy the volume... > > You can boot an older kernel to flush the device without destroying it > (I'm guessing that's because older kernels split down the big requests > which are failing on the 4.4 kernel). Once flushed you could put the > cache into writethrough mode, or use a smaller bucket size.
Indeed, can someone test 4.1.y and see if the problem persists with a 2M bucket size? (If someone has already tested 4.1, then appologies as I've not yet seen that report.) If 4.1 works, then I think a bisect is in order. Such a bisect would at least highlight the problem and might indicate a (hopefully trivial) fix. -- Eric Wheeler > > Tim. > -- > To unsubscribe from this list: send the line "unsubscribe linux-bcache" in > the body of a message to [email protected] > More majordomo info at http://vger.kernel.org/majordomo-info.html > -- dm-devel mailing list [email protected] https://www.redhat.com/mailman/listinfo/dm-devel
