[libvirt-users] libvirt and lvm thin pool

[John Ferlan]

On 08/02/2014 04:24 PM, Vasiliy Tolstov wrote:
> Hi all. I'm using libvirt 1.2.6
> I want to use lvm storage for my virtual machines.
> But i want to use new lvm2 feature - thin pool. How can i do that in
> libvirt? If libvirt can't create it via pool xml, does (and how) to
> use this setup under libvirt?
> 

The 'Thin Pool' is avoided by libvirt, but volumes found a "Sparse" or
"Snapshot" that are part of an lvm thin pool can be utilized by libvirt
- although as the storage administrator understand that if the volume
becomes "filled" - it will go "Inactive"

The key to whether an lvm volume is found is the attributes as seen from
the 'lvs' utility which spits out the following for example:

# lvs
  LV      VG      Attr       LSize   Pool Origin            Data%  Move
Log Cpy%Sync Convert
  home    fedora  -wi-ao---- 136.72g

  root    fedora  -wi-ao----  50.00g

  swap    fedora  -wi-ao----   7.64g

  lv_test lv_pool swi-a-s---   4.00m      [lv_test_vorigin]  50.39

or for a filled and now inactive volume:

# lvs
  LV      VG      Attr       LSize   Pool Origin            Data%  Move
Log Cpy%Sync Convert
  home    fedora  -wi-ao---- 136.72g

  root    fedora  -wi-ao----  50.00g

  swap    fedora  -wi-ao----   7.64g

  lv_test lv_pool swi-I-s---   4.00m      [lv_test_vorigin] 100.00


The 'Attr' column must have an active volume as seen by the 'a' in the
5th field and as Jan points out cannot be a "thin" pool as seen when
there's a 't' in the first column.

Assuming you know have the physical volume already created (via
pvcreate), then a pool definition could be:

# more lv_pool.xml
      <pool type="logical">
        <name>lv_pool</name>
        <source>
          <device path="/dev/sdb2"/>
          <device path="/dev/sdb3"/>
        </source>
        <target>
          <path>/dev/lv_pool</path>
        </target>
      </pool>
#

Where the device path entries would be your physical volumes.

Using 'virsh pool-build lv_pool' and 'virsh pool-start lv_pool' will set
things up allowing for the creation of a volume in the pool such as:

#  more lv_test.xml
<volume>
  <name>lv_test</name>
  <key>r4xkCv-MQhr-WKIT-R66x-Epn2-e8hG-1Z5gY0</key>
  <source>
    <device path='/dev/sdb3'>
    </device>
  </source>
  <capacity>8388608</capacity>
  <allocation>4194304</allocation>
  <target>
    <path>/dev/lv_pool/lv_test</path>
    <permissions>
      <mode>0660</mode>
      <owner>0</owner>
      <group>6</group>
      <label>system_u:object_r:fixed_disk_device_t:s0</label>
    </permissions>
  </target>
</volume>

via 'virsh vol-create lv_pool lv_test.xml' where the 'capacity' and
'allocation' values are the key to determining whether a 'sparse' or
'snapshot' volume from an lvm thin pool will be created. If capacity is
not equal allocation, then the resulting libvirt command adds the
'--virtualsize' (or '--V') to the lvm command used to create the volume
in the pool, e.g.

# /usr/sbin/lvcreate --name lv_test -L 4096K --virtualsize 8192K lv_pool

John