[ovirt-users] Re: Sparse VMs from Templates - Storage issues
On Tue, Aug 10, 2021 at 4:24 PM Shantur Rathore
<shantur.rathore(a)gmail.com> wrote:
Hi all,
I have a setup as detailed below
- iSCSI Storage Domain
- Template with Thin QCOW2 disk
- Multiple VMs from Template with Thin disk
Note that a single template disk used by many vms can become a performance
bottleneck, and is a single point of failure. Cloning the template when creating
vms avoids such issues.
oVirt Node 4.4.4
4.4.4 is old, you should upgrade to 4.4.7.
When the VMs boots up it downloads some data to it and that leads to
increase in volume size.
I see that every few seconds the VM gets paused with
"VM X has been paused due to no Storage space error."
and then after few seconds
"VM X has recovered from paused back to up"
This is normal operation when a vm writes too quickly and oVirt cannot
extend the disk quick enough. To mitigate this, you can increase the
volume chunk size.
Created this configuration drop in file:
# cat /etc/vdsm/vdsm.conf.d/99-local.conf
[irs]
volume_utilization_percent = 25
volume_utilization_chunk_mb = 2048
And restart vdsm.
With this setting, when free space in a disk is 1.5g, the disk will
be extended by 2g. With the default setting, when free space is
0.5g the disk was extended by 1g.
If this does not eliminate the pauses, try a larger chunk size
like 4096.
Sometimes after a many pause and recovery the VM dies with
"VM X is down with error. Exit message: Lost connection with qemu process."
This means qemu has crashed. You can find more info in the vm log at:
/var/log/libvirt/qemu/vm-name.log
We know about bugs in qemu that cause such crashes when vm disk is
extended. I think the latest bug was fixed in 4.4.6, so upgrading to 4.4.7
will fix this issue.
Even with these settings, if you have a very bursty io in the vm, it may
become paused. The only way to completely avoid these pauses is to
use a preallocated disk, or use file storage (e.g. NFS). Preallocated disk
can be thin provisioned on the server side so it does not mean you need
more storage, but you will not be able to use shared templates in the way
you use them now. You can create vm from template, but the template
is cloned to the new vm.
Another option with (still tech preview) is Managed Block Storage (Cinder
based storage). If your storage server is supported by Cinder, we can
managed it using cinderlib. In this setup every disk is a LUN, which may
be thin provisioned on the storage server. This can also offload storage
operations to the server, like cloning disks, which may be much faster and
more efficient.
Nir