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

[Stefano Garzarella]

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.

> 
> 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?

Thanks,
Stefano