linux-ext4 - Re: VMs are pausing with ENOSPC

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Date:	Mon, 11 Nov 2013 10:43:29 +0100
From:	Stefan Hajnoczi <stefanha@...il.com>
To:	linux-ext4@...r.kernel.org
Cc:	kvm@...r.kernel.org, "Brian J. Murrell" <brian@...erlinx.bc.ca>
Subject: Re: VMs are pausing with ENOSPC

On Fri, Nov 08, 2013 at 07:00:32AM -0500, Brian J. Murrell wrote:
> On RHEL 6.4 I have a number of kvms using qemu raw disks.  They are all
> on a filesystem which has reached 500G (of 1TB) of usage.  None of the
> qemu images are full as reported by qemu-img info.  Yet my VMs are all
> pausing with:
> 
> block I/O error in device 'drive-virtio-disk0': No space left on device (28)
> 
> in their logs
> i.e.: # qemu-img info /var/lib/libvirt/images/mgmt1-disk0 
> image: /var/lib/libvirt/images/mgmt1-disk0
> file format: raw
> virtual size: 120G (128849018880 bytes)
> disk size: 52G
> 
> and:
> 
> # df -h /var/lib/libvirt/images/
> Filesystem Size Used Avail Use% Mounted on
> /dev/mapper/vg_00-virt--images 1.0T 500G 473G 52% /var/lib/libvirt/images

Added linux-ext4 mailing list and more info from an IRC debugging
session with Brian:

The image file is on an ext4 file system.  The QEMU userspace process is
performing a pwritev() system call that fails with ENOSPC.

The particular pwritev() that failed had ~600 iovecs covering about 2.4
MB of data.  The file is opened O_DIRECT.

The VMs are all hitting ENOSPC when they write to the file system but
df(1) shows there should still be 48% (~473 GB) available.

Any ideas why writes produce ENOSPC on an ext4 file system with
sufficient space free?

Stefan
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