KVM is supported on the POWER9 LC921 and LC922 systems through the Ubuntu 18.04 and Red Hat Enterprise Linux 7.5 for IBM Power LE (POWER9) distributions.

The following list contains some known issues observed during the extensive testing of KVM on OpenPower POWER9 systems along with some recommendations, best practices, and references:

General issues

Issue: I am seeing guest hangs or timeouts using KVM live migration
Solution: Adjust the migration timeout value (–timeout) to avoid unnecessary downtime.
Guest hangs and file system issues have been observed during live migration. This is still under investigation.

Issue: I’m seeing filesystems being remounted as read-only
Solution: If using Ubuntu guests in POWER8 compatibility mode, then turn transparent huge pages (THP) off. This is still under investigation on the host side.

Tuning

Issue: I’m seeing some performance degradation and instability when using SCSI emulation disks
Solution: Use SCSI passthrough instead of SCSI emulation with disks for better performance and stability. For more information, see Fibre Channel storage compatibility with KVM-QEMU and IBM POWER Systems.

Storage

Issue: I’m seeing poor storage performance
Solution: One possible source of lower than expected storage performance is that the default I/O scheduler algorithm is “cfq”. Consider setting it to use “deadline” on the host for improved performance. For more information, see Using the Deadline IO Scheduler. The same issues exists on Ubuntu.

Issue: I’m finding some issues using the XFS File system in my KVM guests. What is the problem?
Solution: We have observed issues in guests using the XFS file system in certain edge conditions. Please refer to https://bugzilla.redhat.com/show_bug.cgi?id=1549160 for more details. Until this issue is resolved, selecting an alternative filesystem for the guest, such as ext4, can be used to work around the issue.

Issue: I’m attempting a disk hot plug and my guest crashed.
Solution: Trying to remove a disk while it is in use may lead to a guest crash. This happens when trying to add a disk with a duplicate ID. Check to see that your ID for your hotplug is unique. Duplicate IDs can cause the guest to crash.

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