[Xen-devel] qdisks and stubdomains

[Simon Weald <simonw@xxxxxxxxxx>]

Hello

I don't know whether this classifies as a bug or missing functionality,
but I'm looking to attach Ceph RBD volumes directly to a guest (as
opposed to mapping them on the host and then passing the block device to
the guest). The rationale for this is that the kernel RBD client's
functionality lags behind that of librbd, meaning the kernel cannot map
RBD volumes created with the newer features.

For a little bit of background, these are PVHVM guests, I'm running
4.7.1 and 4.8.0 on Jessie, and the Ceph cluster is 10.2.5 (latest Jewel
release).

When running the guest outside of a stubdomain, block-attach exits
successfully:

xl block-attach domU format=raw, vdev=xvdb, access=rw,
backendtype=qdisk, target=rbd:cinder/test:id=xenhosts

root@xen:~# xl block-list domU
Vdev  BE  handle state evt-ch ring-ref BE-path
768   0   6      4     12     8        /local/domain/0/backend/vbd/6/768
5632  0   6      6     -1     -1       /local/domain/0/backend/qdisk/6/5632
832   0   6      3     22     897      /local/domain/0/backend/qdisk/6/832

root@mshx-rd-6:~# xenstore-ls /local/domain/0/backend/qdisk/6/832 -f
/local/domain/0/backend/qdisk/6/832/frontend =
"/local/domain/6/device/vbd/832"
/local/domain/0/backend/qdisk/6/832/params =
"aio:rbd:cinder/test:id=xenhosts"
/local/domain/0/backend/qdisk/6/832/frontend-id = "6"
/local/domain/0/backend/qdisk/6/832/online = "1"
/local/domain/0/backend/qdisk/6/832/removable = "0"
/local/domain/0/backend/qdisk/6/832/bootable = "1"
/local/domain/0/backend/qdisk/6/832/state = "2"
/local/domain/0/backend/qdisk/6/832/dev = "hdb"
/local/domain/0/backend/qdisk/6/832/type = "qdisk"
/local/domain/0/backend/qdisk/6/832/mode = "w"
/local/domain/0/backend/qdisk/6/832/device-type = "disk"
/local/domain/0/backend/qdisk/6/832/discard-enable = "1"
/local/domain/0/backend/qdisk/6/832/feature-flush-cache = "1"
/local/domain/0/backend/qdisk/6/832/feature-persistent = "1"
/local/domain/0/backend/qdisk/6/832/info = "0"
/local/domain/0/backend/qdisk/6/832/feature-discard = "1"
/local/domain/0/backend/qdisk/6/832/hotplug-status = "connected"

This all looks good at this point, however the device doesn't actually
appear available to the guest (no device nodes, nothing in dmesg). This
however is academic, as the ultimate goal is to attach the volumes to
stubdomained guests (without relying on the problematic kernel client
and block scripts). When I try a block-attach to the stubdomained guest,
I get the following:

libxl: error: libxl_dm.c:2423:libxl__dm_check_start: device model
required but not running
libxl: error: libxl.c:2012:device_addrm_aocomplete: unable to add device
libxl_device_disk_add failed.

Is this even possible?

Thanks in advance!

Simon


-- 

Simon Weald
Systems Administrator

PGP: http://www.simonweald.com/SimonWeald.asc
     https://pgp.mit.edu/pks/lookup?op=get&search=0x988E9858747ABE88

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