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
