Re: QEMU RBD is slow with QCOW2 images

[Stefano Garzarella]

On Thu, Mar 04, 2021 at 03:59:17PM +0100, Kevin Wolf wrote:
> Am 04.03.2021 um 15:08 hat Stefano Garzarella geschrieben:
> > 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).
>
> Ugh, I see why. We're passing the protocol driver a QemuOpts that was
> created for a QemuOptsList with the qcow2 default, not for its own
> QemuOptsList. This is wrong.
>
> Note that the QemuOptsList is not qcow2_create_opts itself, but a list
> that is created with qemu_opts_append() to combine qcow2 and rbd options
> into a new QemuOptsList. For overlapping options, the format wins.
>
> I don't think you can change the QemuOptsList of an existing QemuOpts,
> nor is there a clone operation that could just copy all options into a
> new QemuOpts created for the rbd QemuOptsList, so maybe the easiest
> hack^Wsolution would be converting to QDict and back...

Do you mean something like this? (I'll send a proper patch when 
everything is a little clearer to me :-)

diff --git a/block.c b/block.c
index a1f3cecd75..74b02b32dc 100644
--- a/block.c
+++ b/block.c
@@ -671,13 +671,33 @@ out:
 int bdrv_create_file(const char *filename, QemuOpts *opts, Error **errp)
 {
     BlockDriver *drv;
+    QemuOpts *new_opts;
+    QDict *qdict;
+    int ret;

     drv = bdrv_find_protocol(filename, true, errp);
     if (drv == NULL) {
         return -ENOENT;
     }

-    return bdrv_create(drv, filename, opts, errp);
+    if (!drv->create_opts) {
+        error_setg(errp, "Driver '%s' does not support image creation",
+                   drv->format_name);
+        return -ENOTSUP;
+    }
+
+    qdict = qemu_opts_to_qdict(opts, NULL);
+    new_opts = qemu_opts_from_qdict(drv->create_opts, qdict, errp);
+    if (new_opts == NULL) {
+        ret = -EINVAL;
+        goto out;
+    }
+
+    ret = bdrv_create(drv, filename, new_opts, errp);
+out:
+    qemu_opts_del(new_opts);
+    qobject_unref(qdict);
+    return ret;
 }

> > > > 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.
>
> opts->list is already wrong, I think this is what we need to fix.
>
> > > > 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 :-)
>
> I'm almost sure that compatibility will make this more complicated than
> it sounds, but we could have a try.

Eheh I see your point.

Thanks,
Stefano