Re: virtio-blk/ext4 error handling for host-side ENOSPC

[Stefan Hajnoczi <stefanha@redhat.com>]

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> What do you think of this idea? Also, has anything similar been attempted yet?

Hi Keiichi,
Yes, there is an existing approach that is related but not identical to
what you are exploring:

QEMU has an option to pause the guest and raise a notification to the
management tool that ENOSPC has been reached. The guest is unable to
resolve ENOSPC itself and guest applications are likely to fail the disk
becomes unavailable, hence the guest is simply paused.

In systems that expect to hit this condition, this pause behavior can be
combined with an early notification when a free space watermark is hit.
This way guest are almost never paused because free space can be added
before ENOSPC is reached. QEMU has a write watermark feature that works
well on top of qcow2 images (they grow incrementally so it's trivial to
monitor how much space is being consumed).

I wanted to share this existing approach in case you think it would work
nicely for your use case.

The other thought I had was: how does the new ENOSPC error fit into the
block device model? Hopefully this behavior is not virtio-blk-specific
behavior but rather something general that other storage protocols like
NVMe and SCSI support too. That way file systems can handle this in a
generic fashion.

The place I would check is Logical Block Provisioning in SCSI and NVMe.
Perhaps there are features in these protocols for reporting low
resources? (Sorry, I didn't have time to check.)

Stefan

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