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

[Kevin Wolf <kwolf@redhat.com>]

Am 19.04.2019 um 14:23 hat Stefano Garzarella geschrieben:
> Hi Kevin,
> 
> On Wed, Apr 17, 2019 at 10:04:43AM +0200, Kevin Wolf wrote:
> > 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.
> 
> Very interesting, I'll try to add this support also in the rbd driver.

I don't understand - what is the thing you want to add to the rbd driver?
qcow2 doesn't need special protocol driver support for doing this, and I
don't think the QEMU rbd driver has any metadata that could be split off.

> > > > 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
> 
> I did some tests and it seems that the cost of rbd_resize() is
> negligible. IIUC it only updates the metadata without allocating any
> blocks (if we are growing, like that case).
> 
> Anyway the automatic resize will not affect the current use-case (raw
> images on rbd), where the file size is set during the creation, so I
> think there should not be side effects with this patch.

Okay, sounds good.

> I'm also adding the support for preallocation (i.e. full) in the rbd
> driver that can be useful for qcow2 images.
> 
> If you prefer I can resend this patch with the preallocation series.

Let's keep seperate things separate. Huge patch series are always harder
to handle.

Kevin