Hi, everyone. Nice to see this project turning into reality. I will try to spend time here as possible, which is hard with already being spread thinly. I've been a XenServer user for around a decade and am as interesting in learning as well as contributing whatever knowledge might be helpful to the community.

Best regards,
-=Tobias

And from the CLI:

1. xe host-list (to get the UUID of the host)
2. xe pool-eject host-uuid=<host_UUID>

@robyt It depends on (1) licensing, if any, as some licenses go by cores vs. sockets, and (2) NUMA/VNUMA depending on how critical the performance is depending on how the VCPUs get allocated between sockets or on a single socket. Best way IMO is to try all and test with benchmarks. See, for example, this article and the previous two articles, as well as articles by Frank Denneman and others: https://blogs.mycugc.org/2019/04/30/a-tale-of-two-servers-part-3-the-influence-of-numa-cpus-and-sockets-cores-persocket-plus-other-vm-settings-on-apps-and-gpu-performance/

@olivierlambert said in NUMA-impact - Xeon/Epyc - 1P vs 2P:

There is no universal answer (because it's mostly depending on your VM load and what do you expect). As usual, my advice is to keep it simple if you don't have a problem with it (ie: you are satisfied by the perf.). Even a default EPYC configuration will be likely always better than a Xeon one.

After that, if you want to go deeper and learn the details, it's OK, let me just ping @tjkreidl who did a remarkable job (if I remember correctly) on this very topic.

Thanks for the mention, @olivierlambert ! Here's a link to part 3, which contains links back to parts 1 and 2. Note that NUMA will affect EPYC processors differently as they changed the die configuration at one point with the number of cores. I'm open for any questions on this topic. https://blogs.mycugc.org/2019/04/30/a-tale-of-two-servers-part-3-the-influence-of-numa-cpus-and-sockets-cores-persocket-plus-other-vm-settings-on-apps-and-gpu-performance/

@epretorious I would add that you have to be careful about overprovisioning when NUMA/vNUMA kicks in, that is when you allocate more VCPUs to exceed the number of physical CPUs of a bank of them as well as the associated physical memory (assume, for the sake of argument, you have two banks of physical CPUs and each has directly accessible to it one of two banks of memory) then things get inefficient because a CPU may need to go across to a different bank of memory to access data and there is additional overhead involved. See for example this article and the two preceding it:
https://blogs.mycugc.org/2019/04/30/a-tale-of-two-servers-part-3-the-influence-of-numa-cpus-and-sockets-cores-persocket-plus-other-vm-settings-on-apps-and-gpu-performance/

-=Tobias

@olivierlambert Agreed. The Citrix forum used to be very active, but especially since Citrix was taken over, https://community.citrix.com has had way less activity, sadly.
It's still gratifying that a lot of the functionality still is common to both platforms, although as XCP-ng evolves, there will be continually less commonality.

@Chrome Cheers -- always glad to help out. I put in many thousands of posts on the old Citrix XenServer site, and am happy to share whatever knowledge I still have, as long as it's still relevant! In a few years, it probably won't be, so carpe diem!

@Chrome Fantastic! Please mark my post as helpful if you found it as such. Was traveling much of today, hence the late response.

BTW, it's always good to make a backup and/or archive of your LVM configuration anytime you change it, as the restore option is the cleanest way to deal with connectivity issues if there is some sort of corruption. It's saved my rear end before, I can assure you!

Yeah, if the SSD drive got wiped, there's no option to get those back unless you made a backup somewhere of all that before you installed XCP-ng onto it.

BTW, another very useful command for LVM is "vgchange -ay" which will attempt to renew VG information if a VG seems missing or the like.

@Chrome As M. Lambert says, you may be able to sue pbd-plug to re-attach the SR if you can sr-introduce the old SR back into the system.
If not, and if your LVM configuration has not been wiped out, here are some steps t try to recover it (it's an ugly process!):

1. Identify the LVM configuration:
   Check for Backups: Look for LVM metadata backups in /etc/lvm/archive/ or /etc/lvm/backup/.
   Use vgscan: This command will search for volume groups and their metadata.
   Use pvscan: This command will scan for physical volumes.
   Use lvs: This command will list logical volumes and their status.
   Use vgs: This command will list volume groups.
2. Restore from Backup (if available):
   Find the Backup: Locate the LVM metadata backup file (e.g., /etc/lvm/backup/<vg_name>).
   Boot into Rescue Mode: If you're unable to access the system, boot into a rescue environment.
   Restore Metadata: Use vgcfgrestore to restore the LVM configuration.
3. Recreate LVM Configuration (if no backup):
   Identify PVs: Use pvscan to list available physical volumes.
   Identify VGs: Use vgscan to identify volume groups if they are present.
   Recreate PVs: If necessary, use pvcreate to create physical volumes.
   Create VGs: If necessary, use vgcreate to create a new volume group.
   Create LVs: If necessary, use lvcreate to create logical volumes.
4. Mount and Verify:
   Mount the Logical Volumes: Mount the restored LVM volumes to their respective mount points.
   Verify Data: Check the integrity of the data on the restored LVM volumes.
5. Extend LVM (if adding capacity):
   Add a new disk: Ensure the new disk is recognized by the system.
   Create PV: Use pvcreate on the new disk.
   Add PV to VG: Use vgextend to add the PV to the volume group.
   Extend LV: Use lvextend to extend the size of an existing logical volume.
   Extend Filesystem: Use e2resize (for ext4) or resize2fs (for ext3) to extend the filesystem on the LV.

@Chrome Do then just a "xe vm-list" and see if you recogniize any VMs other than the dom0 instance of XCP-ng.
If there is nothing else showing up, you will need to try to find your other LVM storage.

@Chrome Try "xe vm-list params=all"
Do you only have a local storage or did you have any attached storage that's not showing up?

@Chrome OK, now try "xe sr-introduce" (check the syntax for the full command syntax you need), depending on what your connection type is:

xe sr-introduce uuid=<device uuid> shared=true type=lvmohba name-label=<name>

xe sr-introduce uuid=<device uuid> shared=true type=lvmoiscsi name-label=<name>

xe sr-introduce uuid=<device uuid> shared=true type=nfs name-label=<name>

If you are lucky and the interface still exists by which the SR was attached, that might do the trick.

How many hosts in your pool? For HA to work out of the box, you need at least three hosts in a pool. Also, are all your hosts properly time synchronized to the same time source?
They need to be very close in time to each other for HA to work properly. Note that when HA is first enabled on a given host, it has to be rebooted for HA to function.

@lucasljorge That may be the issue. That's pretty full for a coalesce to work!

@utmoab Strange, but I've seen some tasks "stuck" that could not be cancelled for some reason or aanother. A reboot is a drastic measure, but unfortunate;y sometimes the only recourse.

@kevdog You can force a host to be ejected even if the host is not reachable any longer from the pool master using:
xe host-forget uuid=UUID
If there is any issue because VMs are thought to be running still on that host,
you may need to do a power state reset on such before you can get rid of that host.
See if that works for you.

@planedrop CPU compatibility is automatic now, albeit the CPUs have to be somewhat similar. That said, Intel and AMD CPUs cannot be mixed. There used to be scripts out there that you could use to check compatibility but I have not seen them for quite some time. Note that the CPU mask will be reduced to the lowest common denominator -- that is to say, the oldest, least-featured CPU in the pool -- so I would be careful what servers you include in your pool(s).
See also: https://xcp-ng.org/forum/topic/6896/non-server-cpu-compatibility-ryzen-and-intel/21

@olivierlambert @joehays Easiest backup to a remote location IMO is to NFS-mount a drive from some other system and back up to it. That way, it can always be exported to anywhere needed, even if your server(s) are destroyed and you have to start over from scratch.

@jhansen I'd suggest booting off some Linux rescue disk and just redoing all the partitions manually. I ran into an upgrade issue once on a Dell box because of some stupid little extra utility partition they had in there. See: http://xenserver.pl/author/tobias-kreidl
Look for "The (In)famous Dell Utility Partition."

Tapdisk is just palin not good as a storage control mechanism, IMO. There's only so much you can pull out of it.