[libvirt-users] using LVM thin pool LVs as a storage for libvirt guest

[Jan Hutař]

Hello,
as usual, I'm few years behind trends so I have learned about LVM thin
volumes recently and I especially like that your volumes can be "sparse"
- that you can have 1TB thin volume on 250GB VG/thin pool.

Is it somehow possible to use that with libvirt?

I have found this post from 2014:

    https://www.redhat.com/archives/libvirt-users/2014-August/msg00010.html

which says you should be able to create "sparse" volume with `virsh
vol-create ...` or that libvirt should be able to see thin volumes you
create yourself, but neither of that works for me
(libvirt-3.2.1-6.fc26.x86_64).

This way I try to create new:

    # vgs storage
      VG      #PV #LV #SN Attr   VSize   VFree
      storage   1   1   0 wz--n- 267.93g    0 
    # lvs storage -a
      LV              VG      Attr       LSize   Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert
      [lvol0_pmspare] storage ewi-------  68.00m                                                    
      lvol1           storage twi-aotz-- 267.80g             0.00   0.44                            
      [lvol1_tdata]   storage Twi-ao---- 267.80g                                                    
      [lvol1_tmeta]   storage ewi-ao----  68.00m
    # virsh pool-dumpxml storage
    <pool type='logical'>
      <name>storage</name>
      <uuid>f523aed2-a7e4-4dc2-88db-0193a7337704</uuid>
      <capacity unit='bytes'>287687311360</capacity>
      <allocation unit='bytes'>287687311360</allocation>
      <available unit='bytes'>0</available>
      <source>
        <device path='/dev/nvme0n1p3'/>
        <name>storage</name>
        <format type='lvm2'/>
      </source>
      <target>
        <path>/dev/storage</path>
      </target>
    </pool>
    # cat /tmp/big.xml
    <volume>
      <name>big</name>
      <capacity>1073741824</capacity>
      <allocation>1048576</allocation>
      <target>
        <path>/dev/storage/big</path>
      </target>
    </volume>
    # virsh vol-create storage /tmp/big.xml 
    error: Failed to create vol from /tmp/big.xml
    error: internal error: Child process (/usr/sbin/lvcreate --name big -L 1024K --type snapshot --virtualsize 1048576K storage) unexpected exit status 5:   Volume group "storage" has insufficient free space (0 extents): 1 required.

When I create thin volume manually, I do not see it:

    # lvcreate -n big -V 500G --thinpool storage/lvol1
      Using default stripesize 64.00 KiB.
      WARNING: Sum of all thin volume sizes (500.00 GiB) exceeds the size of thin pool storage/lvol1 and the size of whole volume group (267.93 GiB)!
      For thin pool auto extension activation/thin_pool_autoextend_threshold should be below 100.
      Logical volume "big" created.
    # lvs storage -a
      LV              VG      Attr       LSize   Pool  Origin Data%  Meta%  Move Log Cpy%Sync Convert
      big             storage Vwi-a-tz-- 500.00g lvol1        0.00                                   
      [lvol0_pmspare] storage ewi-------  68.00m                                                     
      lvol1           storage twi-aotz-- 267.80g              0.00   0.45                            
      [lvol1_tdata]   storage Twi-ao---- 267.80g                                                     
      [lvol1_tmeta]   storage ewi-ao----  68.00m                                                     
    # virsh vol-list storage
     Name                 Path                                    
    ------------------------------------------------------------------------------


Do I understand the concept incorrectly, or is there something else to
configure?

At the end I want to get max IO performance with possibility to
"overcommit disk space". I know I can use storage "dir", but thought
there might be something faster?

Thank you very much for a response,
Jan



-- 
Jan Hutar     Systems Management QA
jhutar at redhat.com     Red Hat, Inc.