[dm-crypt] bcache gets stuck flushing writeback cache when used in combination with LUKS/dm-crypt and non-default bucket size

Eric Wheeler bcache at lists.ewheeler.net
Fri May 20 01:22:46 CEST 2016


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.
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