Re: Process stuck in md_flush_request (state: D)

[Shaohua Li <shli@kernel.org>]

On Mon, Feb 27, 2017 at 09:49:59AM -0500, Les Stroud wrote:
> After a period of a couple of weeks with one of our test instances having this problem every other day, they were all nice enough to operate without an issue for 9 days.  It finally reoccurred last night on one of the machines.  
> 
> It exhibits the same symptoms and the call traces look as they did previously.  This particular instance is configured with a deadline scheduler.  I was able to capture the inflight you requested:
> 
> $ cat /sys/block/xvd[abcde]/inflight
>        0        0
>        0        0
>        0        0
>        0        0
>        0        0
> 
> I’ve had this happen on instances with the deadline scheduler and the noop scheduler.  At this point, I have not had this happen on an instance that is noop and the raid filesystem (ext4) is mounted with nobarrier.  The instances with noop/nobarrier have not been running long enough for me to make any sort of conclusion that it works around the problem.  Frankly, I’m not sure I understand the interaction between ext4 barriers and raid0 block flushes well enough to theorize whether it should or shouldn’t make a difference.

If nobarrier, ext4 doesn't send flush request.
 
> Does any of this help with identifying the bug?  Is there anymore information I can get that would be useful?  


Unfortunately I can't find anything fishing. Does the xcdx disk correctly
handle flush request? For example, you can do the same test with a single such
disk and check if anything wrong.

Thanks,
Shaohua