On 01/04/2012 11:30 AM, Wido den Hollander wrote:
Hi,
The last few days I've been working on a storage backend driver for
libvirt which supports RBD.
This has been in the tracker for a while:
http://tracker.newdream.net/issues/1422
My current work can be found at: http://www.widodh.nl/git/libvirt.git in
the 'rbd' branch.
Awesome! Glad to see this being worked on.
I realize it is far from done, a lot of work has to be done, but I'd
like to discuss some things first before making some decisions I might
later regret.
My idea was to discuss it here first and after a few iterations get it
reviewed by the libvirt guys.
Let me start with the XML:
<pool type='rbd'>
<name>cephclusterdev</name>
<source>
<name>myrbdpool</name>
<host name='[2a00:f10:11b:cef0:230:48ff:fed3:b086]' port='6789' prefer_ipv6='true'/>
<auth type='cephx' id='admin' secret='a313871d-864a-423c-9765-5374707565e1'/>
</source>
</pool>
I think it will be easier to manage if the format for network volumes
and network disks are as similar as possible. In particular, allowing
multiple hosts, and making the auth element match the network disk
format (even using the same xml schema). With this in mind, the format
would be more like:
<pool type='rbd'>
<name>cephclusterdev</name>
<source name='myrbdpool'>
<host name='[2a00:f10:11b:cef0:230:48ff:fed3:b086]' port='6789'/>
<host name='[2a00:f10:11b:cef0:230:48ff:fed3:b086]' port='6790'/>
<host name='foo.example.org' port='6789'/>
</source>
<auth username='admin'>
<secret type='ceph' uuid='a313871d-864a-423c-9765-5374707565e1'/>
</auth>
</pool>
Or the secret could be identified by name:
<pool type='rbd'>
<name>cephclusterdev</name>
<source name='myrbdpool'>
<host name='[2a00:f10:11b:cef0:230:48ff:fed3:b086]' port='6789'/>
<host name='[2a00:f10:11b:cef0:230:48ff:fed3:b086]' port='6790'/>
<host name='foo.example.org' port='6789'/>
</source>
<auth username='admin'>
<secret type='ceph' usage='mysecretname'/>
</auth>
</pool>
A few things here:
* I'm leaning on the secretDriver from libvirt for storing the actual
cephx key. Should I also store the id in there or keep that in the pool
declaration?
I'd say keep it in the pool declaration for consistency.
* prefer_ipv6? I'm a IPv6 guy, I try to get as much over IPv6 as I can.
Since Ceph doesn't support dual-stack you have to explicitly enable
IPv6. I did not want to let librados read a ceph.conf from outside
libvirt I added this variable. Not the fanciest way I think, but it
could serve other future storage drivers in libvirt
This actually isn't necessary for RBD - the ms_bind_ipv6 option only
affects servers (who call bind(2)).
* How should we pass other configuration options? I want to stay away
from the ceph.conf as far as possible. Imho a user should be able to
define a XML and get it all up and running. You will also run into
apparmor/SELinux on systems, so libvirt won't have permission to read
files everywhere you want it to. I also thinks the libvirt guys want to
keep everything as generic as possible.
I agree, libvirt should be able to configure everything with no external
files.
In the future we might see more
storage backends which have almost the same properties as RBD. How do we
pass extra config options? the volume
The libvirt way seems to be adding more well-defined elements or
attributes to the xml schema when the new backend is added. Personally
I'd be happy with a generic <option>:<value> mapping, but I don't think
libvirt devs would like that. But this doesn't really matter for the
pool implementation - all the info we need to connect is well-defined in
the disk xml.
That's the XML file for declaring the pool.
The pool itself uses librados/librbd instead of invoking the 'rbd' command.
The other storage backends do invoke external binaries, but that didn't
seem the right way here since we have the luxury of C-API's.
I'm aware of the fact that a lot of memory handling and cleaning won't
be as it should be. I'm fairly new to C, so I'll make mistakes here and
there.
The current driver is however focused on Qemu/KVM, since that is
currently the only virtualization technique which supports RBD.
This exposes another problem. Then you do a "dumpxml" it expects a
target path which is up until now an absolute path to a file or block
device.
Recently disks with the type 'network' were introduced for Sheepdog and
RBD, but attaching a 'network' volume to a domain is currently not
possible with the XML schemes. I'm thinking about a generic way to
attach network volumes to a domain.
It seems like RBD will need to provide the full information (image,
hosts, and username/secret) to be able to attach a volume. Maybe this
should go in the volume xml? The libvirt devs probably have a good idea
of the right approach here. It looks like programs using libvirt will
have to adjust for this, but libvirt itself doesn't know how to attach a
volume to a guest.
Another feature I'd like to add in the future is managing kernel RBD. We
could set up RBD for the user and mapping and unmapping devices on
demand for virtual machines.
The 'rbd' binary does this mapping, but that is done in the binary
itself and not by librbd. Would it be a smart move to add a map() and
unmap() method to librbd?
I'm not sure this should go in librbd - I'd rather make the 'rbd' binary
more usable for mapping/unmapping without any ceph.conf.
The last thing I'm thinking about is the spare allocation of the RBD
images. Right now both 'allocation' and 'capacity' are set to the
virtual size of the RBD image. rbd_stat() does not report the actual
size of the image, it only reports the virtual size of the image. Is
there a way to figure out how big a RBD image actually is?
There's no way to do this efficiently right now. It is possible to add
an allocation bitmap, and we might as an optimization for layering, but
that's farther down the road.
My plan is to add RBD support to CloudStack after the libvirt
integration has finished. CloudStack heavily relies on the storage pools
of libvirt, so adding RBD support to CloudStack depends on libvirt.
Feedback is welcome on this!
Thanks,
Wido
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