Ditto for Qcow2. We use it on ZFS.

I do a snapshot-create-as to get the QCOW2 consistent image, then zfs snapshot, then replicate to another host for backups. All the VMS have consistent backups in this way.

Please don't use lvm thin for vm. In our hosting in Russia we have
100-150 vps on each node with lvm thin pool on ssd and have locks,
slowdowns and other bad things because of COW. After we switching to
qcow2 files on plain ssd ext4 fs and happy =).


2017-11-04 23:21 GMT+03:00 Jan Hutař <jhutar@redhat.com>:

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@redhat.com     Red Hat, Inc.

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