Re: [PATCH] rbd: replace the rbd sysfs interface

[Greg KH <greg@kroah.com>]

On Mon, Nov 22, 2010 at 04:48:54PM -0800, Yehuda Sadeh Weinraub wrote:
> On Mon, Nov 22, 2010 at 4:14 PM, Greg KH <greg@kroah.com> wrote:
> > On Fri, Nov 19, 2010 at 12:42:51PM -0800, Yehuda Sadeh Weinraub wrote:
> >> On Thu, Nov 18, 2010 at 6:08 PM, Greg KH <greg@kroah.com> wrote:
> >> > On Thu, Nov 18, 2010 at 02:53:35PM -0800, Yehuda Sadeh Weinraub wrote:
> >> >> Yes, pretty much. One problem that I do see is that if we define the
> >> >> snaps/ as a device (and not just as a kobj) as you suggested before,
> >> >> it'll automatically create a 'uevent' entry under it which can be a
> >> >> real issue in the case we have a snapshot named like that. Shouldn't
> >> >> we just create it as a kobj in that case?
> >> >
> >> > No.  Just use the subdirectory option of an attribute group to handle
> >> > that and you will not need to create any device or kobject with that
> >> > name, the driver core will handle it all automatically for you.
> >> >
> >>
> >> One issue with using the groups name, is that it's not nested (unless
> >> I'm missing something), so we can't have it done for the entire
> >> planned hierarchy without holding a kobject on the way. Just a
> >> reminder, the device-specific hierarchy would look like this:
> >>
> >> 1. /sys/bus/rbd/devices/<id>/
> >> 2. /sys/bus/rbd/devices/<id>/<device_attrs>
> >> 3. /sys/bus/rbd/devices/<id>/snaps/
> >> 4. /sys/bus/rbd/devices/<id>/snaps/<snap_name>/
> >> 5. /sys/bus/rbd/devices/<id>/snaps/<snap_name>/<snap_attrs>
> >>
> >> One solution would be to create kobjects for (3) and for (4), without
> >> using a group name.
> >
> > Ick, no.
> >
> >> Another way, we can create groups for (2), and (3)
> >> under (1), but that's about it,
> >
> > attribute group for 2 is fine.
> >
> >> you can't create the snap specific directory this way without
> >> resorting to some internal sysfs directory creation, which will be
> >> horribly wrong. At that point we don't have anything for 'snaps', and
> >> we don't really need to do any operations under that directory, we
> >> just need it to exist so that it contains the snapshot-specific
> >> directories.
> >
> > But you need to do something with those snapshots, right?  So, why even
> > have "snaps" be a subdir?  Why not just make <snap_name> a struct device
> > with <id> being the parent, and it living on the same bus_type, but
> > being a different device_type (like partitions and block devices are),
> 
> The reason we keep snapshots in a separate subdirectory is that they
> can have arbitrary name. So either we prefix them and put them in a
> common namespace with the devices, or we put them down the hierarchy.

Do either one.  I would suggest a prefix.

> In any case we don't do any operations on them, we just have them for
> informational use and we put them there so that we don't have one big
> file that lists them all.

But something cares about them, so treat them properly.

> >> Another way would be to create a group for (2) under (1) and create a
> >> kobject for (3), for which you can create group per snapshot.
> >>
> >> Am I missing something? We already have the first solution (kobjects
> >> only) implemented, is there some real benefit for using the third
> >> method? We'll have to manually add remove groups anyway, as snapshots
> >> can be removed and new snapshots can be added.
> >
> > Never add kobjects to a struct device, that is showing you that
> > something is wrong, and that userspace really will want to get that
> > create/destroy event of the sub child.
> >
> 
> But they're there as information device attributes, it's nothing like
> partitions in block devices. So we want to just be able to list them
> and their attributes easily (and nicely), without having to put them
> in one big file.

Then use a prefix and put everything in the same subdirectory underneath
the id and you should be fine, right?

thanks,

greg k-h