Re: QEMU RBD is slow with QCOW2 images

[Stefano Garzarella]

On Thu, Mar 04, 2021 at 01:05:02PM +0100, Kevin Wolf wrote:
> Am 03.03.2021 um 18:40 hat Stefano Garzarella geschrieben:
> > Hi Jason,
> > as reported in this BZ [1], when qemu-img creates a QCOW2 image on RBD
> > writing data is very slow compared to a raw file.
> >
> > Comparing raw vs QCOW2 image creation with RBD I found that we use a
> > different object size, for the raw file I see '4 MiB objects', for 
> > QCOW2 I
> > see '64 KiB objects' as reported on comment 14 [2].
> > This should be the main issue of slowness, indeed forcing in the code 4 MiB
> > object size also for QCOW2 increased the speed a lot.
> >
> > Looking better I discovered that for raw files, we call rbd_create() with
> > obj_order = 0 (if 'cluster_size' options is not defined), so the default
> > object size is used.
> > Instead for QCOW2, we use obj_order = 16, since the default 'cluster_size'
> > defined for QCOW2, is 64 KiB.
>
> Hm, the QemuOpts-based image creation is messy, but why does the rbd
> driver even see the cluster_size option?
>
> The first thing qcow2_co_create_opts() does is splitting the passed
> QemuOpts into options it will process on the qcow2 layer and options
> that are passed to the protocol layer. So if you pass a cluster_size
> option, qcow2 should take it for itself and not pass it to rbd.
>
> If it is passed to rbd, I think that's a bug in the qcow2 driver.

IIUC qcow2 properyl remove it, but when rbd uses 
qemu_opt_get_size_del(opts, BLOCK_OPT_CLUSTER_SIZE, 0) the default value 
of qcow2 format is returned.

Going in depth in qemu_opt_get_size_helper(), I found that 
qemu_opt_find() properly returns a NULL pointer, but then we call 
find_default_by_name() that returns the default value of qcow2 format 
(64k).

> > Using '-o cluster_size=2M' with qemu-img changed only the qcow2 cluster
> > size, since in qcow2_co_create_opts() we remove the 'cluster_size' from
> > QemuOpts calling qemu_opts_to_qdict_filtered().
> > For some reason that I have yet to understand, after this deletion, however
> > remains in QemuOpts the default value of 'cluster_size' for qcow2 (64 KiB),
> > that it's used in qemu_rbd_co_create_opts()
>
> So it seems you came to a similar conclusion. We need to find out where
> the 64k come from and just fix that so that rbd uses its default.

Yes, I tried debugging above, but I'm not sure how to fix it.

Maybe a new parameter in qemu_opt_get_size_helper() to prevent it from 
looking for the default value.
Or we should prevent the default value from being added to the 
opts->list->desc, but that part is still not very clear to me.

> > At this point my doubts are:
> > Does it make sense to use the same cluster_size as qcow2 as object_size in
> > RBD?
> > If we want to keep the 2 options separated, how can it be done? Should we
> > rename the option in block/rbd.c?
>
> My lazy answer is that you could just use QMP blockdev-create, where you
> create layer by layer separately.
>
> What could possibly be done for the QemuOpts is using the dotted syntax
> like for opening, so you could specify file.cluster_size=... for the
> protocol layer (or data_file.cluster_size=... for the external data
> file etc.)

This would be cool :-)

Thanks,
Stefano