Re: [Qemu-devel] [RFC 2/5] block: Generic truncation fallback

[Kevin Wolf <kwolf@redhat.com>]

Am 11.07.2019 um 21:58 hat Max Reitz geschrieben:
> If a protocol driver does not support truncation, we call fall back to
> effectively not doing anything if the new size is less than the actual
> file size.  This is what we have been doing for some host device drivers
> already.

Specifically, we're doing it for drivers that access a fixed-size image,
i.e. block devices rather than regular files. We don't want to do this
for drivers where the file size could be changed, but just didn't
implement it.

So I would suggest calling the function more specifically something like
bdrv_co_truncate_blockdev(), and not using it as an automatic fallback
in bdrv_co_truncate(), but just make it the BlockDriver.bdrv_co_truncate
implementation for those drivers where it makes sense.

And of course, we only need these fake implementations because qemu-img
(or .bdrv_co_create_opts) always wants to create the protocol level. If
we could avoid this, then we wouldn't need any of this.

Kevin