[RFC] rbd sysfs interface

[Yehuda Sadeh Weinraub]

The rbd module that was recently merged into the linux kernel on the
2.6.37-rc1 merge window is based on the osdblk device driver. Other
than being somewhat confusing (as in ceph we use the term osd too) it
had been a valuable resource and jump started the development effort.
One thing that we inherited from osdblk is the sysfs class interface
that was used there. Generally, there is a single flat class control
directory that allows adding, removing and listing of devices. For rbd
we expanded this interface to include all snapshots operations too.
This, however, might not be completely suitable to rbd. First, there
might be many different devices, so that having a single control for
all is cumbersome. The problem is exacerbated with rbd snapshots, as
there can be many snapshots to a single device, so using a single
control for all devices and snapshots doesn't scale.

Another point to consider is the integration with udev, so that
devices can be created and mapped automaticallty to /dev.
We'd like to replace the current rbd class interface, and if possible
to make it asap, so that the current interfaces won't be set in stone
once 2.6.37 is out. At this point we wanted to do something like the
following:

Under /sys/class/rbd there'd we'll keep the 'add' entry that adds rbd devices:

 # echo "10.0.0.1 name=admin rbd myimage" > /sys/class/rbd/add

The devices that'll be created still be enumerated, and there'll be a
subdirectory under rbd/ for each (actually a soft link to
/sys/devices/virtual/rbd/<id>). For each device we'll have multiple
read-only properties (name, pool, size, client_id, major, cur_snap,
snapshots) and a few control entries that'll allow controlling it
(e.g., remove, refresh, snap_create, snap_rollback).

We're not sure whether the available snapshots go under the device
(e.g., rbd/<id>/snaps/...) or just keep it on a single 'snapshots'
entry.

Does this seem sane? Any comments would be greatly appreciated.

Yehuda
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