> Yes, on file based storage a snapshot is a file, and it grows as > needed. On block based > storage, a snapshot is a logical volume, and oVirt needs to extend it > when needed.
Forgive my ignorance, coming from vSphere background where a filesystem was created on iSCSI LUN. I take that this isn't the case in case of a iSCSI Storage Domain in oVirt. On Wed, Aug 11, 2021 at 12:26 PM Nir Soffer <[email protected]> wrote: > On Wed, Aug 11, 2021 at 12:43 AM Shantur Rathore > <[email protected]> wrote: > > > > Thanks for the detailed response Nir. > > > > In my use case, we keep creating VMs from templates and deleting them so > we need the VMs to be created quickly and cloning it will use a lot of time > and storage. > > That's a good reason to use a template. > > If your vm is temporary and you like to drop the data written while > the vm is running, you > could use a temporary disk based on the template. This is called a > "transient disk" in vdsm. > > Arik, maybe you remember how transient disks are used in engine? > Do we have an API to run a VM once, dropping the changes to the disk > done while the VM was running? > > > I will try to add the config and try again tomorrow. Also I like the > Managed Block storage idea, I had read about it in the past and used it > with Ceph. > > > > Just to understand it better, is this issue only on iSCSI based storage? > > Yes, on file based storage a snapshot is a file, and it grows as > needed. On block based > storage, a snapshot is a logical volume, and oVirt needs to extend it > when needed. > > Nir > > > Thanks again. > > > > Regards > > Shantur > > > > On Tue, Aug 10, 2021 at 9:26 PM Nir Soffer <[email protected]> wrote: > >> > >> On Tue, Aug 10, 2021 at 4:24 PM Shantur Rathore > >> <[email protected]> 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 > >> > >
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