Using URI aliases with libvirt 0.9.7 (forthcoming release)

Posted: October 28th, 2011 | Filed under: Fedora, libvirt, Virt Tools | Tags: , , , | 2 Comments »

In the next week or two, we will be releasing libvirt 0.9.7, which comes with a new URI alias feature, which will be particularly useful for users of tools like virsh and virt-install.

In the same way that SSH allows you to setup hostname aliases in $HOME/.ssh/config, libvirt will now allow you to setup URI aliases in $HOME/.libvirt/libvirt.conf (if you are running unprivileged) or /etc/libvirt/libvirt.conf (if you are running as root). NB do not confuse this file with libvirtd.conf which is a server side libvirtd daemon config file.

The new libvirt.conf configuration file will start with a single ‘uri_aliases’ parameter, which takes a list of entries. Each entry is an alias map in the format “ALIAS=URI”. To avoid potential overlap with otherwise valid URIs, the ALIAS part is restricted to the characters a-z, A-Z, 0-9, -, _. The config file format is fairly self-explanatory if you look at an example:

$ cat $HOME/.libvirt/libvirt.conf
uri_aliases = [

Once this config has been created, I can then replace any commands like

$ virsh -c qemu+ssh:// list
$ virt-install -c qemu+tls:// ...some args...

with much simpler commands like

$ virsh -c hail list
$ virt-install -c sleet ...some arg...

Application that use libvirt do not have to do anything special to support URI aliases, as they are automatically handled by the virConnectOpen family of APIs. If, however, an application wishes to prevent the use of URI aliases for some reason, it can do so by passing VIR_CONNECT_NO_ALIASES constant as a flag to the virConnectOpenAuth API.

For further documentation on libvirt URIs consult the URI reference and the remote driver reference pages on the libvirt website.

Debugging early startup of KVM with GDB, when launched by libvirtd

Posted: October 12th, 2011 | Filed under: Fedora, libvirt, Virt Tools | Tags: , , , , | 1 Comment »

Earlier today I was asked how one would go about debugging early startup of KVM under GDB, when launched by libvirtd. It was not possible to simply attach to KVM after it had been launched by libvirtd, since that was too late. In addition running the same KVM command outside libvirt did not exhibit the problem that was being investigated.

Fortunately, with a little cleverness, it is actually possible to debug a KVM guest launched by libvirtd, right from the start. The key is to combine a couple of breakpoints with use of follow-fork-mode. When libvirtd starts up a KVM guest, it runs QEMU a couple of times in order to detect which command line arguments are supported. This means the follow-fork-mode setting cannot be changed too early, otherwise GDB will end up following the wrong process.

I happen to know that there is only one place in the libvirt code which calls virCommandSetPreExecHook, and that is immediately before launching the real QEMU process. A nice thing about GDB is that when following forked/exec’d children, it will apply any existing breakpoints in the child, even if it is a new binary. So a break point set on ‘main’, while still in libvirtd will happily catch ‘main’ in the QEMU process. The only remaining problem is that if QEMU does not setup and activate the monitor quickly enough, libvirtd will try to kill it off again. Fortunately GDB lets you ignore SIGTERM, and even SIGKILL :-)

The start of the trick is this:

# pgrep libvirtd
# gdb
(gdb) attach 12345
(gdb) break virCommandSetPreExecHook
(gdb) cont

Now in a separate shell

# virsh start $GUESTNAME

Back in the GDB shell the breakpoint should have triggered, allowing the trick to be finished:

(gdb) break main
(gdb) handle SIGKILL nopass noprint nostop
Signal        Stop	Print	Pass to program	Description
SIGKILL       No	No	No		Killed
(gdb) handle SIGTERM nopass noprint nostop
Signal        Stop	Print	Pass to program	Description
SIGTERM       No	No	No		Terminated
(gdb) set follow-fork-mode child
(gdb) cont
process 3020 is executing new program: /usr/bin/qemu-kvm
[Thread debugging using libthread_db enabled]
[Switching to Thread 0x7f2a4064c700 (LWP 3020)]
Breakpoint 2, main (argc=38, argv=0x7fff71f85af8, envp=0x7fff71f85c30)
    at /usr/src/debug/qemu-kvm-0.14.0/vl.c:1968
1968	{

Bingo, you can now debug QEMU startup at your leisure

Setting up a Ceph cluster and exporting a RBD volume to a KVM guest

Posted: October 12th, 2011 | Filed under: Fedora, libvirt, Virt Tools | Tags: , , , , , , , | 5 Comments »

Yesterday I talked about setting up Sheepdog with KVM, so today is it is time to discuss use of Ceph and RBD with KVM.

Host Cluster Setup, the easy way

Fedora has included Ceph for a couple of releases, but since my hosts are on Fedora 14/15, I grabbed the latest ceph 0.3.1 sRPMs from Fedora 16 and rebuilt those to get something reasonably up2date. In the end I have the following packages installed, though to be honest I don’t really need anything except the base ‘ceph’ RPM:

# rpm -qa | grep ceph | sort

Installing the software is the easy bit, configuring the cluster is where the fun begins. I had three hosts available for testing all of which are virtualization hosts. Ceph has at least 3 daemons it needs to run, which should all be replicated across several hosts for redundancy. There’s no requirement to use the same hosts for each daemon, but for simplicity I decided to run every Ceph daemon on every virtualization host.

My hosts are called lettuce, avocado and mustard. Following the Ceph wiki instructions, I settled on a configuration file that looks like this:

    auth supported = cephx
    keyring = /etc/ceph/keyring.admin

    keyring = /etc/ceph/keyring.$name
    host = lettuce
    host = avocado
    host = mustard

    osd data = /srv/ceph/osd$id
    osd journal = /srv/ceph/osd$id/journal
    osd journal size = 512
    osd class dir = /usr/lib64/rados-classes
    keyring = /etc/ceph/keyring.$name
    host = lettuce
    host = avocado
    host = mustard

    mon data = /srv/ceph/mon$id
    host = lettuce
    mon addr =
    host = avocado
    mon addr =
    host = mustard
    mon addr =

The osd class dir bit should not actually be required, but the OSD code looks in the wrong place (/usr/lib instead of /usr/lib64) on x86_64 arches.

With the configuration file written, it is time to actually initialize the cluster filesystem / object store. This is the really fun bit. The Ceph wiki has a very basic page which talks about the mkcephfs tool, along with a scary warning about how it’ll ‘rm -rf’ all the data on the filesystem it is initializing. It turns out that it didn’t mean your entire host filesystem, AFAICT, it only the blows away the contents of the directory configured for ‘osd data‘ and ‘mon data‘, in my case both under /srv/ceph.

The recommended way is to let mkcephfs ssh into each of your hosts and run all the configuration tasks automatically. Having tried the non-recommended way and failed several times before finally getting it right, I can recommend following the recommended way :-P There are some caveats not mentioned in the wiki page though:

  • The configuration file above must be copied to /etc/ceph/ceph.conf on every node before attempting to run mkcephfs.
  • The configuration file on the host where you run mkcephfsmust be in /etc/ceph/ceph.conf or it will get rather confused about where it is in the other nodes.
  • The mkcephfscommand must be run as root since, it doesn’t specify ‘-l root’ to ssh, leading to an inability to setup the nodes.
  • The directories /srv/ceph/osd$i must be pre-created, since it is unable to do that itself, despite being able to creat the /srv/ceph/mon$idirectories.
  • The Fedora RPMs have also forgotten to create /etc/ceph

With that in mind, I ran the following commands from my laptop, as root

 # n=0
 # for host in lettuce avocado mustard ; \
   do \
       ssh root@$host mkdir -p /etc/ceph /srv/ceph/mon$n; \
       n=$(expr $n + 1; \
       scp /etc/ceph/ceph.conf root@$host:/etc/ceph/ceph.conf
 # mkcephfs -a -c /etc/ceph/ceph.conf -k /etc/ceph/keyring.bin

On the host where you ran mkcephfs there should now be a file /etc/ceph/keyring.admin. This will be needed for mounting filesystems. I copied it across to all my virtualization hosts

 # for host in lettuce avocado mustard ; \
   do \
       scp /etc/ceph/keyring.admin root@$host:/etc/ceph/keyring.admin; \

Host Cluster Usage

Assuming the setup phase all went to plan, the cluster can now be started. A word of warning though, Ceph really wants your clocks VERY well synchronized. If your NTP server is a long way away, the synchronization might not be good enough to stop Ceph complaining. You really want a NTP server on your local LAN for hosts to sync against. Sort this out before trying to start the cluster.

 # for host in lettuce avocado mustard ; \
   do \
       ssh root@$host service ceph start; \

The ceph tool can show the status of everything. The ‘mon’, ‘osd’ and ‘msd’ lines in the status ought to show all 3 host present & correct

# ceph -s
2011-10-12 14:49:39.085764    pg v235: 594 pgs: 594 active+clean; 24 KB data, 94212 MB used, 92036 MB / 191 GB avail
2011-10-12 14:49:39.086585   mds e6: 1/1/1 up {0=lettuce=up:active}, 2 up:standby
2011-10-12 14:49:39.086622   osd e5: 3 osds: 3 up, 3 in
2011-10-12 14:49:39.086908   log 2011-10-12 14:38:50.263058 osd1 197 : [INF] 2.1p1 scrub ok
2011-10-12 14:49:39.086977   mon e1: 3 mons at {0=,1=,2=}

The cluster configuration I chose has authentication enabled, so to actually mount the ceph filesystem requires a secret key. This key is stored in the /etc/ceph/keyring.admin file that was created earlier. To view the keyring contents, the cauthtool program must be used

# cauthtool -l /etc/ceph/keyring.admin 
	key = AQDLk5VOeHkHLxAAfGjcaUsOXOhJr7hZCNjXSQ==
	auid = 18446744073709551615

The base64 key there will be passed to the mount command, repeating on every host needing a filesystem present:

# mount -t ceph /mnt/ -o name=admin,secret=AQDLk5VOeHkHLxAAfGjcaUsOXOhJr7hZCNjXSQ==
error adding secret to kernel, key name client.admin: No such device

For some reason, that error message is always printed on my Fedora hosts, and despite that, the mount has actually succeeded

# grep /mnt /proc/mounts /mnt ceph rw,relatime,name=admin,secret= 0 0

Congratulations, /mnt is now a distributed filesystem. If you create a file on one host, it should appear on the other hosts & vica-verca.

RBD Volume setup

A shared filesystem is very nice, and can be used to hold regular virtual disk images in a variety of formats (raw, qcow2, etc). What I really wanted to try was the RBD virtual block device functionality in QEMU. Ceph includes a tool called rbd for manipulating those. The syntax of this tool is pretty self-explanatory

# rbd create --size 100 demo
# rbd ls
# rbd info demo
rbd image 'demo':
	size 102400 KB in 25 objects
	order 22 (4096 KB objects)
	block_name_prefix: rb.0.0
	parent:  (pool -1)

Alternatively RBD volume creation can be done using qemu-img …. at least once the Fedora QEMU package is fixed to enable RBD support.

# qemu-img create -f rbd rbd:rbd/demo  100M
Formatting 'rbd:rbd/foo', fmt=rbd size=104857600 cluster_size=0 
# qemu-img info rbd:rbd/demo
image: rbd:rbd/foo
file format: raw
virtual size: 100M (104857600 bytes)
disk size: unavailable

KVM guest setup

The syntax for configuring a RBD block device in libvirt, is very similar to that used for Sheepdog. In Sheepdog, every single virtualization node is also a storage node, so there is no hostname required. Not so for RBD. Here it is necessary to specify one or more host names, for the RBD servers.

<disk type='network' device='disk'>
  <driver name='qemu' type='raw'/>
  <source protocol='rbd' name='demo/wibble'>
    <host name='' port='6798'/>
    <host name='' port='6798'/>
    <host name='' port='6798'/>
  <target dev='vdb' bus='virtio'/>

More observant people might be wondering how QEMU gets permission to connect to the RBD server, given that the configuration earlier enabled authentication. This is thanks to the magic of the /etc/ceph/keyring.admin file which must exist on any virtualization server. Patches are currently being discussed which will allow authentication credentials to be set via libvirt, avoiding the need to store the credentials on the virtualization hosts permanently.

A blog planet for following virtualization tools development & tutorials, etc

Posted: October 11th, 2011 | Filed under: Fedora, libvirt, Virt Tools | Tags: , , , , , | 2 Comments »

The website, launched last year, provides tutorials, videos, documentation, online help and roadmaps relevant to libvirt, libguestfs, gtk-vnc, spice, other related libraries, and tools or applications like virt-manager & virt-install. The site goal is to inform & assist end users, system administrators & application developers who wish to learn about the capabilities of the virt tools stack. The focus of most content is the, state of the art, Linux native KVM hypervisor, but writing about using other hypervisors using virt tools is also welcome.

Back in June I finally got around to setting up a blog planet to aggregate the RSS feeds of various people working in libvirt, libguestfs, etc. While I announced this to various mailing lists, it appears I forgot to blog about it. Whoops. So this post is just a quick alert that if you’re interested in libvirt, libguestfs, virt-manager, etc and don’t want to follow a high traffic site like the Fedora planet, then this is the blog feed aggregator for you:

Setting up a Sheepdog cluster and exporting a volume to a KVM guest

Posted: October 11th, 2011 | Filed under: Fedora, libvirt, Virt Tools | Tags: , , , , , | 3 Comments »

There were recently patches posted to libvir-list to improve the Ceph support in the KVM driver. While trying to review them it quickly became clear I did not have enough knowledge of Ceph to approve the code. So I decided it was time to setup some clustered storage devices to test libvirt with. I decided to try out Ceph, GlusterFS and Sheepdog, and by virtue of Sheepdog compiling the fastest, that is the first one I have tried and thus responsible for this blog post.

Host setup

If you have Fedora 16, sheepdog can directly installed using yum

# yum install sheepdog

Sheepdog relies on corosync to maintain cluster membership, so the first step is to configure that. Corosync ships with an example configuration file, but since I’ve not used it before, I chose to just use the example configuration recommended by the Sheepdog website. So on the 2 hosts I wanted to participate in the cluster I created:

# cat > /etc/cluster/cluster.conf <EOF
compatibility: whitetank
totem {
  version: 2
  secauth: off
  threads: 0
  interface {
    ringnumber: 0
    bindnetaddr: -YOUR IP HERE-
    mcastport: 5405
logging {
  fileline: off
  to_stderr: no
  to_logfile: yes
  to_syslog: yes
  logfile: /var/log/cluster/corosync.log
  debug: off
  timestamp: on
  logger_subsys {
    subsys: AMF
    debug: off
amf {
  mode: disabled

Obviously remembering to change the ‘bindnetaddr‘ parameter. One thing to be aware of is that this configuration allows any host in the same subnet to join the cluster, no authentication or encryption is required. I believe corosync has some support for encryption keys, but I have not explored this. If you don’t trust the network, this should definitely be examined. Then it is simply a matter of starting the corosync and sheepdog, each on each node:

# service corosync start
# service sheepdog start

If all went to plan, it should be possible to see all hosts in the sheepdog cluster, from any node:

# collie node list
   Idx - Host:Port              Number of vnodes
     0 -    	64
*    1 -    	64

The final step in initializing the nodes is to create a storage cluster across the nodes. This command only needs to be run on one of the nodes

# collie cluster format --copies=2
# collie cluster info

Ctime                Epoch Nodes
2011-10-11 10:50:01      1 [,]

Volume setup

libvirt has a storage management API for creating/managing volumes, but there is not currently a driver for sheepdog. So for the time being, volumes need to be created manually using the qemu-img command. All that is required is a volume name and a size. So on any of the nodes:

$ qemu-img create sheepdog:demo 1G

The more observant people might notice that this command can be run by any user on the host, no authentication required. Even if the host is locked down to not allow unprivileged user logins, this still means that any compromised QEMU instance can access all the sheepdog storage. Not cool. Some form of authentication is clearly needed before this can be used for production.

With the default Fedora configuration of sheepdog, all the disk volumes end up being stored under /var/lib/sheepdog, so make sure that directory has plenty of free space.

Guest setup

Once a volume has been created, setting up a guest to use it, is just a matter of using a special XML configuration block for the guest disk.

<disk type='network' device='disk'>
  <driver name='qemu' type='raw'/>
  <source protocol='sheepdog' name='demo'/>
  <target dev='vdb' bus='virtio'/>

Notice how although this is a network block device, there is no need to provide a hostname of the storage server. Every virtualization host is a member of the storage cluster, and vica-verca, so the storage is “local” as far as QEMU is concerned. Inside the guest there is nothing special to worry about, a regular virtio block device appears, in this case /dev/vdb. As data is written to the block device in the guest, the data should end up in /var/lib/sheepdog on all nodes in the cluster.

One final caveat to mention, is that live migration of guests between hosts is not currently supported with Sheepdog.

Edit: Live migration *is* supported with sheepdog 0.2.0 and later.