> 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. To help narrow this down, I tested the following generic pre-compiled mainline kernels on Ubuntu 15.10: * WORKS: http://kernel.ubuntu.com/~kernel-ppa/mainline/v4.3.6-wily/ * DOES NOT WORK: http://kernel.ubuntu.com/~kernel-ppa/mainline/v4.4-rc1+cod1-wily/ I also tried the default & latest distribution-provided 4.2 kernel. It worked. This one also worked: * WORKS: http://kernel.ubuntu.com/~kernel-ppa/mainline/v4.2.8-wily/ So it seems to me that it is a regression from 4.3.6 kernel to any 4.4 kernel. That should help save time with bisection... James -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
