Known Issues
This article contains advisories and minor issues in the XenServer 9 release and any workarounds that you can apply.
General
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When attempting to use the serial console to connect to a XenServer® host, the serial console might refuse to accept keyboard input. If you wait until after the console refreshes twice, the console then accepts keyboard input.(CA-311613)
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After a License Activation Service (LAS) disconnection, a XenServer host using online activation might remain in the grace license period even after the LAS becomes reachable again. The host continues to report
LICENSE_SERVER_UNREACHABLEandGRACE_LICENSEalerts despite successfully communicating with the LAS. This issue affects only hosts configured for online activation; hosts using offline activation are unaffected. To work around this issue, restart the toolstack on each affected host. (XSI-2204) -
After applying the June 2026 XenServer updates to hosts originally installed from a XenServer 9 Preview ISO dated May 2026 or earlier, some VM lifecycle operations might fail with an internal error. Affected operations include suspend, checkpoint, import, and storage live migration. This issue occurs because applying updates installs a new kernel but does not reboot the host, so the host continues to run the previous kernel. To work around this issue, reboot the host after applying the update. Hosts installed from June 2026 (or later) ISOs are not affected. (CA-428974)
Guests
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When defining the vCPU topology for a new VM, ensure that the number of cores per socket is set to a power of two (1, 2, 4, 8, and so on). If this value is not set to a power of two, some of your cores are not utilized by the VM, topology information shown inside the VM operating system might be incorrect, and other unexpected behaviors might be observed.(XSI-1766)
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If your VM enters sleep mode, your mouse might become unresponsive although your keyboard remains functional. Rebooting the VM or disabling and enabling the USB controller through Device Manager restores mouse functionality.(CA-399746)
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If a VM is assigned multiple NVIDIA T4 vGPUs, attempts to suspend (or live migrate) the VM might fail.(CA-428435)
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If you assign multiple USB passthrough devices to a VM, the VM might fail to start.(CA-428550)
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On a VM that uses BIOS boot mode, the GRUB boot menu does not respond to keystrokes from the VM console. You cannot use the arrow keys to change the highlighted entry, interrupt the countdown, or select a non-default entry such as a rescue entry. The VM boots the default entry when the countdown ends. This issue affects both newly created and imported BIOS-mode VMs. VMs that use UEFI boot mode are not affected, and the console responds normally after the VM has booted. Where possible, create or import VMs in UEFI boot mode to avoid this issue.(CA-428924)
Performance
- NUMA node drift: when VMs fully use all CPUs on the host, then the hypervisor may run some vCPUs on NUMA nodes far from where the VM’s memory is located, causing a performance degradation. Use the
NUMA non-affine vCPUsRRD (XenCenter Performance Graph) to monitor whether you are affected by this, and try to leave at least 1 CPU idle to recover. (XSI-1573)
Windows guests
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For domain-joined Windows 10 VMs (1903 and later) with FireEye Agent installed, repeated successful RDP connections can cause the VM to freeze with 100% CPU usage in
ntoskrnl.exe. Perform a hard reboot on the VM to recover from this state. (CA-323760) -
When you create a UEFI VM, the Windows installation requires a key press to start. If you do not press a key during the required period, the VM console switches to the UEFI shell.
To work around this issue, you can restart the installation process in one of the following ways:
- In the UEFI console, type the following commands:
EFI:
EFI\BOOT\BOOTX64
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- Reboot the VM
When the installation process restarts, watch the VM console for the installation prompt. When the prompt appears, press any key.(CA-333694)
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If you boot a large number of Windows Server 2025 VMs on a single host at the same time, some might fail to start.(CP-51655)
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If an imported Windows VM already has XenServer VM Tools installed, attempting to install the same version of the tools again during the import process fails.(CA-413698)
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If you import a Windows Server VM with more than 4 disks, some of the disks show as offline.(CA-414047)
For details on the latest XenServer VM Tools for Windows release, see What’s new.
Linux guests
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You cannot use the Dynamic Memory Control (DMC) feature on Red Hat Enterprise Linux 8, Red Hat Enterprise Linux 9, Red Hat Enterprise Linux 10 (preview), Rocky Linux 8, Rocky Linux 9, CentOS Stream 9, or CentOS Stream 10 VMs as these operating systems do not support memory ballooning with the Xen® hypervisor.(CA-378797)
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On some Linux VMs, especially busy systems with outstanding disk I/O, attempts to suspend or live migrate the VM might fail. To work around this issue, try increasing the value of
/sys/power/pm_freeze_timeout, for example, to 300000. If this work-around is not successful, you can upgrade the Linux kernel of the VM to the latest version.(CP-41455)
Storage
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You cannot run more than 200 PVS-Accelerator-enabled VMs on a XenServer host.(CP-39386)
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When read caching is enabled, it may be slower to read from the parent snapshot than from the leaf. To work around this issue, allocate more memory to dom0.(CP-32853)
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Sometimes, when the most recent snapshot for a VM is deleted, it is not possible to completely merge the data from the active VM disk into its parent device. Often, this is caused by the VM being active and writing data to the disk faster than the merge process can merge it. As a result, the VM might use more storage space than is usually necessary due to having two virtual disks in a delta parent/child arrangement. This issue can have more impact on LVM-based SR types as the VMs active write disk is fully provisioned. To allow for the data merge process to complete, either
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Temporarily reduce the storage I/O rate that the VM is producing.
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Shut down the VM.
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Then rescan the SR on which the VM’s disks are stored.(XSI-1922)
Third-party
- A limitation in recent SSH clients means that SSH does not work for user names that contain any of the following characters:
{}[]|&. Ensure that your user names and Active Directory server names do not contain any of these characters.
XenCenter
For information about known and fixed issues in XenCenter, see XenCenter What’s New.