(co-written with @dthenot)
QCOW2 RC2 release notes
Hello everyone,
We’re happy to publish the second and last release candidate for QCOW2 support in XCP-ng 8.3, before general availability.
It allows to use the QCOW2 open format (from Qemu) for virtual disks (VDIs) instead of the VHD format, and overcome the limitations imposed by the latter. The most important one being a size limit of 2 TiB per virtual disk.
Adding support for QCOW2 to XCP-ng 8.3 has been a 1.5 year journey, mobilizing many developers of the XCP-ng team.
There was a double challenge :
- Add the new feature, for those among you who really need large disks.
- Maintain the strong reliability of the current VHD support, that is what you are using now in production.
We managed to offer support for this new format without requiring to destroy and re-create your existing storage repositories. However, adding this QCOW2 support had a large impact on the codebase. More than you would usually want on a LTS product.
That’s why we invested a lot of energy, time, and resources on QA. So that we can offer this feature without affecting XCP-ng’s stability.
We need the final touch now, that is: feedback from the community.
The most important test is also the simplest: update your labs (and/or less important pools) from the testing repository, and verify that it all works as expected. VMs, snapshots, live migration, making backups, restoring backups… You don’t need to actually use the new QCOW2 format for your tests to be extremely useful.
Then, you can also start using the new disk format if you wish so (see below).
The new packages versions are:
For those of you who took part in the previous betas and RC
First, a big thank you! Then, important information about this RC2.
End of the dedicated repository
You can now remove the dedicated repository file, /etc/yum.repos.d/xcp-ng-qcow2.repo. Future updates will follow the normal pattern.
VHD by default
You will notice one major difference: the default for image format has gone back to being VHD, rather than QCOW2.
Your QCOW2 VDIs will still work after the update, but, by default, migrating a VDI to a SR will try to create a VHD instead of a QCOW2 (and will cause migration to fail if the VDI is bigger than 2TiB).
You can change this behaviour by defining QCOW2 as the preferred image format for the target SR. See below.
Image format management (VHD, QCOW2)
This update introduces the concept of image formats. The two possible image formats are now vhd (our historical format) and qcow2.
- Each storage repository (except LINSTOR and SMB) supports both image formats.
- SRs will create any new disk as VHD by default, in order to retain the same behaviour as before the update.
Exception: Xen Orchestra will automatically try to create a QCOW2 disk if the virtual size is bigger than 2TiB.
- During a storage migration from a SR to another SR, the destination format is chosen by the destination SR, following the same rules as for the creation of a new disk (this can be used to convert from one format to another).
If you want all new VDIs to be created as QCOW2 disks on a SR, you can set its preferred-image-formats on your SR.
Configuring a SR’s preferred-image-formats
New SRs
Configuring the preferred image format for new SRs can be done in Xen Orchestra’s SR creation form.
You can also still add the parameter at the SR creation on the command line with xe: device-config:preferred-image-formats=qcow2
Example:
xe sr-create name-label="test-lvmsr" type=lvm device-config:device=/dev/nvme1n1 device-config:preferred-image-formats=qcow2
Existing SRs
To tell an existing SR that it must prefer the qcow2 image format for new disks, it is necessary to unplug, destroy, recreate and re-plug its PBD with the added parameter in the device-config: https://docs.xcp-ng.org/storage/#-how-to-modify-an-existing-sr-connection
In order to unplug the PBD, any VMs with a VDI on the SR will have to be stopped, or the VDI moved temporarily to another SR.
This operation will not affect the contents of the SR. The PDB object represents the connection to the SR.
Why is it plural?
Don’t mind the s at the end of preferred-image-formats for now. At the moment, only the first element of the list is used in most cases.
One exception: if you define the preferred image formats as vhd, qcow2 and attempt to create a new disk with size > 2TiB, Xen Orchestra will automatically select QCOW2 as the format. This is the default for SRs without a configured preferred image format.
Creating a QCOW2 Virtual Disk (VDI) directly
Without changing the preferred image format of the whole SR, you can also directly create a QCOW2 VDI. This is not exposed in Xen Orchestra yet.
Use the xe vdi-create command with sm-config:image-format=qcow2.
Example:
xe vdi-create sr-uuid=<SR UUID> virtual-size=5TiB name-label="My QCOW2 VDI" sm-config:image-format=qcow2
What’s interesting to know
What is notable:
- The current maximum limit is 16 TiB per VDI. We could technically go beyond, but we’re only testing up to 16 TiB at the moment.
- We are using the default cluster size from Qemu, which is 64KiB. It is possible to create a VDI with bigger cluster size, see the FAQ for info.
- Of interest to the users of the
largeblock SR, QCOW2 is working on drives with blocksize >512B. The limitation for devices with blocksize of 4KiB is only for VHD. As such it is possible to use a normal SR instead of largeblock if you configure preferred-image-formats to be QCOW2.
- For backups jobs containing QCOW2 images, NBD need to be active on the backup job in XO.
What is not supported:
- QCOW2 image format for LINSTOR SRs (XOSTOR)
- QCOW2 image format for SMB SRs
What is coming soon:
- A way to select the destination format for a migration is being added in XAPI. Currently, the format is only decided by the preferred format of the destination SR
- We have ongoing work to improve performances of the storage stack in general (not just for QCOW2).
Known issues
- Snapshot of RAW VDIs are not working (if working it would create a VHD or QCOW2 image with the parent being the RAW VDI)
- Migrating a > 2TiB VDI towards a SR whose preferred image format is not
qcow2 (it will attempt to create a VDI, and fail)
- We have identified a problem with BIOS VMs when the boot disk is almost exactly 2TiB, 4TiB, 8TiB or 16TiB big. Having the disk being 1MiB bigger or smaller will allow the VM to boot. If you encounter this issue, resizing again with a bit more (minimum 1MiB) should make the disk boot again.
How to install
The update is provided as a regular update candidate in the testing repository. There are other update candidates being published at the same time. You can see what's in the update by looking at the announcement: https://xcp-ng.org/forum/topic/9964/xcp-ng-8-3-updates-announcements-and-testing/432
You can update from the testing repository:
yum clean metadata --enablerepo=xcp-ng-testing,xcp-ng-candidates
yum update --enablerepo=xcp-ng-testing,xcp-ng-candidates
A reboot is necessary after the update.
Time window for the tests
We’re aiming for a general release in about two weeks, maybe three. Given the tight timeline, your feedback will be especially valued!
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2017 Citrix Systems, Inc. For other
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