Re: [Qemu-devel] [PATCH RFC 1/1] block/rbd: increase dynamically the image size

[Kevin Wolf]

Am 17.04.2019 um 09:34 hat Stefano Garzarella geschrieben:
> On Mon, Apr 15, 2019 at 10:04:52AM +0200, Kevin Wolf wrote:
> > 
> > I think a potential actual use case could be persistent dirty bitmaps
> > for incremental backup. Though maybe that would be better served by
> > using the rbd image just as a raw external data file and keeping the
> > qcow2 metadata on a filesystem.
> 
> Thanks to point it out! I'll take a look to understand how to keep
> metadata separated from the data.

I'd consider the feature still experimental, but for local files, it
works like this:

    qemu-img create -f qcow2 -o data_file=test.raw test.qcow2 4G

And then just use test.qcow2. As long as you can put everything you need
into an rbd URL, the same approach should work. Otherwise, you may need
to use QMP blockdev-create on creation and possibly the data-file option
of the qcow2 driver for opening.

> > How fast is rbd_resize()? Does automatically resizing for every write
> > request actually work reasonably well in practice? If it does, there is
> > probably little reason not to allow it, even if the use cases are rather
> > obscure.
> 
> I'll try to measure the percentage of the time spent in the rbd_resize.
> 
> Another solution could be to pass to the rbd driver the virtual size of
> the image and resize it only one time also if the preallocation is
> disabled, because RBD will not allocate blocks but IIUC it only set the max
> size.
> 
> Do you think make sense? Is it feasible?

In theory yes, though it requires modification of every driver that
should be usable together with rbd (i.e. ideally all of the drivers). If
automatic resize works good enough, I'd prefer that.

Kevin