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Xen Ceph Support

The goal of this project is to work towards a widely available, simple and fast integration of Ceph in Xen. The most direct way towards this is via a custom block script in /etc/xen/scripts. The best-performing way would be a LIBRADOS based blockback driver, which is more complex to tackle. Starting there would result in yet another powerful but non-upstreamed dead Xen addon. Instead we go for the simplest option first and will use the time gained to press for upstreaming.

Instead we went for what will immediately work for everyone:

  1. RDB Image
  2. block-rbd script
  3. disk specification

=> domU running natively on Ceph

What's working:

Mapping of a RBD image as specified in the domU config file. Tested to be bootable etc.

Config:

Syntax example:

disk = [ 'script=block-rbd,vdev=xvdN,target=poolname:image_name' ]

To define xvda as im ceph_test in pool rbd :

disk = [ 'script=block-rbd,vdev=xvda,target=rbd:ceph-test' ]

PV-Grub config:

Syntax example for grub0.97:

kernel = "/usr/lib/xen/boot/pv-grub-x86_64.gz"
extra  = "(hd0,0)/boot/grub/menu.lst"

Warning:

PV-Grub can only boot "legacy Grub". On distros using Grub2 (Wheezy, RHEL7) you need to create a valid menu.lst

What's not working:

  • PyGrub The following Xen-devel thread explains the issue: http://thr3ads.net/xen-devel/2013/01/2281686-pygrub-hvm-boot-with-alternate-script-for-block-devices LibXL should attach RBD device and pass it over correctly as "write_dev" stores it... (in xenstore we should params = poolname:image and image as passed to pygrub should be /dev/rbd/pool/image) Right now it looks like PyGrub has never worked with external block device scripts. Who knows. This sucks because PV-Grub has a lot of other limitations

  • Attaching multiple devices - Unexpected, but there's a bug when specifying multiple disks

What's planned:

  • PyGrub support
  • PV-Grub support

Optional:

  • Mild auto-search of images - if name matches the domU name, it might be fair game. Probably no good since most domU have multiple block devices.

  • Being able to auto-clone an image off a base rbd image This could be used to implement VDI autocloning using Xen. It would be implemented as an optional feature triggered via extra flags.

  • Implement proper support for w,r,r!,w! by asking Ceph about single-/multiattachment (so, cluster-wide locking) This could be done nicely with rbd map, which has locking support. The xen hotplug scripts would need "help" to correctly handle it. Right now they only know how to look at single-dom0 level and don't do it for external block scripts. (actually, seems file: type only?)

Wishlist, but would need contributors

  • Librados blockback (would work best, maybe fastest, storage domU support)
  • qdisk Librados (has better SCSI emulation)

Other solutions

blktap / RBD (not very active):

info: http://thr3ads.net/xen-devel/2013/04/2289675-Xen-blktap-driver-for-Ceph-RBD-Anybody-wants-to-test-p source: https://github.com/smunaut/blktap/tree/rbd

Authors:

  • Thomas Zelch
  • Florian Heigl