Re: blockdev driver=file,filename=/dev/block forbidden?

[Daniel P . Berrangé]

On Wed, Sep 29, 2021 at 02:21:58PM +0300, Michael Tokarev wrote:
> Commit 8d17adf34f501ded65a106572740760f0a75577c
> "block: remove support for using "file" driver with block/char devices"
> explicitly forbids usage of file driver for block devices.
> 
> But _why_?
> 
> Hasn't we always used file for everything on *nix? And what's the _actual_
> difference between file and host_device?

There are various things QEMU does differently for plain files vs
block devices. The check for read-only volume, determining data
transfer size, media geometry, supporting discard. This is seen
in the BlockDriver structs that have different callbacks for each.

> Please note this change has been added to qemu long before libvirt has
> been adapted (I guess there's no released libvirt can be used with
> qemu 6.0+).

Can you show your libvirt guest XML config that is causing trouble, as
it has done the right thing here for  a long while AFAIK, so I suspect
a mis-configuration.

Regards,
Daniel
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