Re: [Qemu-devel] [PATCH RFC 0/8] scsi-disk: Active/passive ALUA support

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Fri, Nov 27, 2015 at 03:58:58PM +0100, Hannes Reinecke wrote:
> here's now an updated version to enable ALUA and simplified
> active/passive multipath support for qemu.
> 
> This patchset relies on having _two_ block devices configured,
> and two SCSI disks pointing to those block devices with the
> _same_ 'wwn' property and unique 'port_group' properties.
> I know, this is a bit of a nasty hack, but I hope to add
> proper multipath support (with several SCSI devices pointing /
> linking to the same block device) in the near future.
> 
> It also implements a 'alua_policy', which allows for simulating
> an 'active/passive' multipath setup.
> 
> And for testing I've implemented a 'block_disconnect' HMP command,
> which simulates a link failure for the attached devices.
> 
> I wouldn't object if someone declares this a gross hack, but with
> it I can finally simulate real-life multipath failover and do
> some functional multipath-tools testing withouth having to recurse
> on using real hardware.

I'm not familiar with how ALUA works but have been thinking about a
multipath problem:

If the host has SCSI disks that are marked 'offline' then QEMU will
refuse to start up since it cannot open the block device (ENXIO).

Does it make sense to allow guests to start in this condition?

I think we'd need to notice when the disk comes back online and notify
the guest.

Stefan

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