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    Second (and final) Release Candidate for QCOW2 image format support

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    • stormiS Offline
      stormi Vates 🪐 XCP-ng Team
      last edited by dthenot

      Here's a work in progress version of the FAQ that will go with the release.

      QCOW2 FAQ

      What storage space available do I need to have on my SR to have large QCOW2 disks to support snapshots?

      Depending on a thin or thick allocated SR type, the answer is the same as VHD.
      A thin allocated is almost free, just a bit of data for the metadata of a few new VDI.

      For thick allocated, you need the space for the base copy, the snapshot and the active disk.

      Must I create new SRs to create large disks?

      No. Most existing SR will support QCOW2. LinstorSR and SMBSR (for VDI) does not support QCOW2.

      Can we have multiples different type of VDIs (VHD and QCOW2) on the same SR?

      Yes, it’s supported, any existing SR (unless unsupported e.g. linstor) will be able to create QCOW2 beside VHD after installing the new sm package

      What happen in Live migration scenarios?

      preferred-image-formats on the PBD of the master of a SR will choose the destination format in case of a migration.
      source preferred-image-format VHD or no format specified preferred-image-format qcow2
      qcow2 >2 TiB X qcow2
      qcow2 <2 TiB vhd qcow2
      vhd vhd qcow2

      Can we create QCOW2 VDI from XO?

      XO hasn’t yet added the possibility to choose the image format at the VDI creation.
      But if you try to create a VDI bigger than 2TiB on a SR without any preferred image formats configuration or if preferred image formats contains QCOW2, it will create a QCOW2.

      Can we change the cluster size?

      Yes, on File based SR, you can create a QCOW2 with a different cluster size with the command:

      qemu-img create -f qcow2 -o cluster_size=2M $(uuidgen).qcow2 10G
xe sr-scan uuid=<SR UUID> # to introduce it in the XAPI

      The qemu-img command will print the name, the VDI is <VDI UUI>.qcow2 from the output.

      We have not exposed the cluster size in any API call, which would allow you to create these VDIs more easily.

      Can you create a SR which only ever manages QCOW2 disks? How?

      Yes, you can by setting the preferred-image-formats parameter to only qcow2.

      Can you convert an existing SR so that it only manages QCOW2 disks? If so, and it had VHDs, what happens to them?

      You can modify a SR to manage QCOW2 by modifying the preferred-image-formats parameter of the PBD’s device-config.

      Modifying the PBD necessitates to delete it and recreate it with the new parameter. This implies stopping access to all VDIs of the SR on the master (you can for shared SR migrate all VMs with VDIs on other hosts in the pool and temporarily stop the PBD of the master to recreate it, the parameter only need to be set on the PBD of the master).

      If the SR had VHDs, they will continue to exist and be usable but won’t be automatically transformed in QCOW2.

      Can I resize my VDI above 2 TiB?

      A disk in VHD format can’t be resized above 2 TiB, no automatic format change is implemented.
      It is technically possible to resize above 2 TiB following a migration that would have transferred the VDI to QCOW2.

      Is there any thing to do to enable the new feature?

      Installing updated packages that supports QCOW2 is enough to enable the new feature (packages: xapi, sm, blktap). Creating a VDI bigger than 2 TiB in XO will create a QCOW2 VDI instead of failing.

      Can I create QCOW2 disks lesser than 2 TiB?

      Yes, but you need to create it manually while setting sm-config:image-format=qcow2 or configure preferred image formats on the SR.

      Is QCOW2 format the default format now? Is it the best practice?

      We kept VHD as the default format in order to limit the impact on production. In the future, QCOW2 will become the default image format for new disks, and VHD progressively deprecated.

      What’s the maximum disk size?

      The current limit is set to 16 TiB. It’s not a technical limit, it’s a limit that we corresponds to what we tested. We will raise it progressively in the future.

      We’ll be able to go up to 64 TiB before meeting a new technical limit related to live migration support, that we will adress at this point.

      The theoretical maximum is even higher. We’re not limited by the image format anymore.

      Can I import without modification my KVM QCOW2 disk in XCP-ng?

      No. You can import them, but they need to be configured to boot with the drivers like in this documentation: https://docs.xcp-ng.org/installation/migrate-to-xcp-ng/#-from-kvm-libvirt
      You can just skip the conversion to VHD.

      So it should work depending on different configuration.

      1 Reply Last reply Reply Quote 6
      • A Online
        abudef @stormi
        last edited by

        @stormi said:

        This is not exposed in Xen Orchestra yet.

        When will it likely be available in the GUI?

        stormiS julienXOvatesJ 2 Replies Last reply Reply Quote 0
        • stormiS Offline
          stormi Vates 🪐 XCP-ng Team @abudef
          last edited by

          @abudef I don't know. I forwarded the question.

          1 Reply Last reply Reply Quote 0
          • pkgwP Online
            pkgw
            last edited by

            After upgrading from the QCOW2 beta to this set of packages, I'm running into a pretty severe bug: my QCOW2 disks still exist and are available, but have largely disappeared from the XO UI and many of the lower-level tools.

            In the XO5 storage UI, the disks appear, but their names and descriptions are lost, and even though they are currently attached to running VMs, the system doesn't recognize this:

            141397a1-4770-409a-b431-3a42f824f7c4-image.jpeg

            If I try to assign a new name to one of the VDIs as an experiment, I get a "VDI does not exist" error, even though xe vdi-list does show all of these VDIs.

            On the other hand, I have running VMs where QCOW2 VDIs are attached and mounted, but the xe vm-disk-list command doesn't show them.

            I see that there's a new batch of updates from a few days ago — any chance they will address this?

            stormiS 2 Replies Last reply Reply Quote 0
            • julienXOvatesJ Offline
              julienXOvates Vates 🪐 XO Team @abudef
              last edited by

              @abudef we'll add it in the near-term (June hopefully) as QCOW2 support is a major update in the VMS stack !

              1 Reply Last reply Reply Quote 1
              • stormiS Offline
                stormi Vates 🪐 XCP-ng Team @pkgw
                last edited by

                @pkgw Would it be possible to open a ticket and a support tunnel so that @Team-Storage can look at it?

                1 Reply Last reply Reply Quote 0
                • stormiS Offline
                  stormi Vates 🪐 XCP-ng Team @pkgw
                  last edited by

                  @pkgw Our initial theory is that you might have applied updates at some point which had replaced the sm package with one that didn't support qcow2. Then a next update would have brought it back, but the metadata lost.

                  pkgwP 1 Reply Last reply Reply Quote 0
                  • stormiS Offline
                    stormi Vates 🪐 XCP-ng Team
                    last edited by stormi

                    I just published, in the xcp-ng-testing repository, what is hopefully the very last round of fixes before the feature goes live.

                    You’ll have about three days to share your feedback if you’d like to be part of this final sprint 🏃.

                    Details at https://xcp-ng.org/forum/post/104961

                    1 Reply Last reply Reply Quote 1
                    • pkgwP Online
                      pkgw @stormi
                      last edited by

                      @stormi That is quite possible. I'll open a ticket for further investigation.

                      1 Reply Last reply Reply Quote 2
                      • stormiS Offline
                        stormi Vates 🪐 XCP-ng Team
                        last edited by

                        This is it, it's now out!

                        https://xcp-ng.org/blog/2026/05/05/qcow2-is-now-ga-in-xcp-ng/

                        1 Reply Last reply Reply Quote 1

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