Re: bcache hangs on writes, recovers after disabling discard on cache device

[Juha Aatrokoski <jha-6fQLAAUwtFMxHbG02/KK1g@public.gmane.org>]

On Tue, 16 Jul 2013, Kent Overstreet wrote:

> On Tue, Jul 16, 2013 at 09:14:09PM +0300, Juha Aatrokoski wrote:
>> On Fri, 12 Jul 2013, Juha Aatrokoski wrote:
>>>> Can you give this patch a try? It's on top of the current
>>>> bcache-for-3.11 branch
>>>
>>> OK, now running the same kernel with this patch applied and
>>> discard enabled. However, it has previously taken my system 2-4
>>> days to trigger this bug, so I'd say at least two weeks before I
>>> can say the patch (may have) fixed the issue.
>>
>> No such luck, hit the bug after four days of uptime. Disabling
>> discard fixed the problem so at least it's not any worse than
>> before.
>
> Argh, damn peculiar bug... and the fact that it takes so long to trigger
> is frustrating. I'm honestly at a loss at this point as to what that IO
> actually is.

One thing I noticed is that your patch only affects the allocator, the 
journal still does discards the old way. Perhaps it's worth a try to 
apply a similar change to the journal discards?

> I should've thought of this sooner, but if there's any way you could
> capture some blktrace output while it's doing its spinning thing that
> could be very helpful.

OK, I will attempt to reproduce the bug again after the weekend and I'll 
try to remember to use blktrace.