Re: QEMU RBD is slow with QCOW2 images

[Jason Dillaman]

On Wed, Mar 3, 2021 at 12:41 PM Stefano Garzarella <sgarzare@redhat.com> wrote:
>
> 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.
>
> 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()
>
> At this point my doubts are:
> Does it make sense to use the same cluster_size as qcow2 as object_size
> in RBD?

No, not really. But it also doesn't really make any sense to put a
QCOW2 image within an RBD image. To clarify from the BZ, OpenStack
does not put QCOW2 images on RBD, it converts QCOW2 images into raw
images to store 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?

You can already pass overrides to the RBD block driver by just
appending them after the
"rbd:<filename>[:option1=value1[:option2=value2]]" portion, perhaps
that could be re-used.

> Thanks,
> Stefano
>
> [1] https://bugzilla.redhat.com/show_bug.cgi?id=1744525
> [2] https://bugzilla.redhat.com/show_bug.cgi?id=1744525#c14
>


-- 
Jason