XFS: possible memory allocation deadlock in kmem_alloc

[Dave Chinner]

On Mon, May 30, 2016 at 12:45:07PM +0800, baotiao wrote:
> This machine is running for qemu,  the file is qemu qcow file type
> 
> [root at w-openstack20 /data/nova/instances/7898b630-c4ef-49ab-9ce9-3735e090c282]# file disk
> disk: QEMU QCOW Image (v3), has backing file (path /data/nova/instances/_base/3f27393376152ac352b2d85703011e38517e), 429496729600 bytes

Oh, you're using delta/snapshot based qcow images. That, by it's
very nature, generates fragmented image files as they are a delta
over the backing file.

....
> actual 48421430, ideal 20716, fragmentation factor 99.96%
> 
> How can I solve this problem, what do you suggest me to do?

Use a extent size hint (say 1-8MB) for your qcow2 image files so that
they don't fragment badly as they are written to. 

Cheers,

Dave.
-- 
Dave Chinner
david at fromorbit.com