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discard option on ext4 mounts in QEMU qcow2 images
by Nate Collins 25 Apr '22

25 Apr '22

Hello, Does anyone have any real-world statistics on the reliability and performance of the ext4 discard option in QEMU VMs? I've heard it's discouraged for physical devices, as some are inefficient in how TRIM requests are processed, but I was wondering if there were any drawbacks/warnings when it comes to virtual disks (in this case, QCOW2s). I currently use either virtio-scsi or virtio-blk drivers for disk images depending on what the host/guest support, and run TRIMs weekly, but having an immediate freeing of space is desirable for many reasons. Also wondering if snapshots on snapshotting filesystems (such as ZFS) can run into corruption or other issues if snapshots are taken during a TRIM. Thanks.

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race condition? virsh migrate --copy-storage-all
by Valentijn Sessink 20 Apr '22

20 Apr '22

Hi list, I'm trying to migrate a few qemu virtual machines between two 1G ethernet connected hosts, with local storage only. I got endless "error: operation failed: migration of disk vda failed: Input/output error" errors and thought: something wrong with settings. However, then, suddenly: I succeeded without changing anything. And, hey: while ! time virsh migrate --live --persistent --undefinesource --copy-storage-all ubuntu20.04 qemu+ssh://duikboot/system; do a=$(( $a + 1 )); echo $a; done ... retried 8 times, but then: success. This smells like a race condition, doesn't it? A bit weird is the fact that the migration seems to succeed every time while copying from revolving disks to SSD; but the other way around has this Input/output error. There are some messages in /var/log/syslog, but not at the time of the failure, and no disk errors. These disks are LVM2 volumes and they live on raid arrays - and/so there is not a real, as in physical, I/O-error. Source system has SSD's, target system has regular disks. 1) is this the right mailing list? I'm not 100% sure. 2) how can I research this further? Spending hours on a "while / then" loop to try and retry live migration looks like a dull job for my poor computers ;-) Best regards, Valentijn

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Virtio-scsi and block mirroring
by Bjoern Teipel 19 Apr '22

19 Apr '22

Hello everyone, I’m looking at an issue where I do see guests freezing (Dl) process state during a block disk mirror from one storage to another storage (NFS) where the network stack of the guest can freeze for up to 10 seconds. Looking at the storage and IO I noticed good throughput ad low latency <3ms and I am having trouble to track down the source for the issue, as neither storage nor networking show issues. Interestingly when I do the same test with virtio-blk I do not really see the process freezes at the frequency or duration compared to virtio-scsi which seem to indicate a client side rather than storage side problem. I had looked at the syscalls and nothing stuck out: % time seconds usecs/call calls errors syscall ------ ----------- ----------- --------- --------- ---------------- 28.51 20.672654 8339 2479 ioctl 27.81 20.162714 3379 5967 31 futex 22.02 15.964498 785 20335 poll 15.22 11.038403 150 73561 io_submit 4.17 3.023285 41 73540 lseek 1.20 0.868003 5 158591 write 0.63 0.459030 11 42871 ppoll 0.22 0.159263 8 19314 recvmsg 0.16 0.115520 5 22526 read 0.04 0.029149 29149 1 restart_syscall 0.01 0.009252 28 330 sendmsg 0.00 0.001221 1221 1 munmap 0.00 0.000458 22 21 fcntl 0.00 0.000286 95 3 openat 0.00 0.000166 5 32 rt_sigprocmask 0.00 0.000103 10 10 fdatasync 0.00 0.000099 25 4 clone 0.00 0.000081 7 12 mmap 0.00 0.000077 19 4 close 0.00 0.000076 6 12 mprotect 0.00 0.000056 14 4 madvise 0.00 0.000025 6 4 set_robust_list 0.00 0.000023 6 4 prctl ------ ----------- ----------- --------- --------- ---------------- 100.00 72.504442 419626 31 total Does anyone have an idea how to better debug this issue ? Thanks Bjoern

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When does the balloon-change or control-error event occur
by Han Han 19 Apr '22

19 Apr '22

Hi developers, I have questions about balloon-change or control-error event: 1. What's the meaning of these events 2. When do the events occur? The comments of their callbacks don't mention that( https://gitlab.com/libvirt/libvirt/-/blob/master/include/libvirt/libvirt-do… https://gitlab.com/libvirt/libvirt/-/blob/master/include/libvirt/libvirt-do… )

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Help with libvirt
by Eduardo Kiassucumuca 19 Apr '22

19 Apr '22

Good morning I'm Eduardo, a computer science student and I'm doing a final course work focused on virtualization. The work consists of creating virtual machines on a server and allowing ssh access to the virtual machines that are on the server containing qemu/kvm/libvirt. The problem is that I can't access the virtual machines from an external network but I can access them inside the server. I would like to know what would be the best way since we want to have a single public ip and be able to have a reverse proxy to access the virtual machines, I would like to know from your experience what you recommend?

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Libvirt vs QEMU benefits
by M, Shivakumar 19 Apr '22

19 Apr '22

Hello, For one of our case validation, we were using direct QEMU commands before for VM creation as it was easier to configure the VMs. Inside VM we do run the real-time latency test. Recently we switched to libvirt for the VM creation and deletion. Surprisingly, we do see a significant increase in the real-time latency performance for the VMs launched through the libvirt. W.r.t configuration wise both VMs are the same, we just converted the existing QEMU commands into libvirt XMLs. I am wondering what and all the features which libvirt has, improving this performance. Thanks, Shivakumar M

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Libvirt Daemon Question
by Michael Espinoza 13 Apr '22

13 Apr '22

Hi I’ve been googling this for a bit with no luck. Is there a way of determining what mode an already running libvirt daemon is running in.(session or system) From the docs it sounded like it was determined by what user started the daemon. But in a test node I set up, I have libvirtd running as root yet I can still connect to both the session and system qemus with virsh. And if I were to want libvirtd only in session mode, how would I do that? Thanks for the help! This stuff is really awesome and I’m just trying to get a full understanding of it. -Mike -- Michael Espinoza Mobile Phone : 646 327 9273 Email : Mikespinoza(a)gmail.com

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"default" watchdog device - ?
by lejeczek 08 Apr '22

08 Apr '22

Hi guys. Without explicitly, manually using watchdog device for a VM, the VM (centOS 8 Stream 4.18.0-365.el8.x86_64) shows '/dev/watchdog' exists. To double check - 'dumpxml' does not show any such device - what kind of a 'watchdog' that is? watchdog-5.15-2.el8.x86_64 on such a VM does not seem to do anything with it. Host is centOS 9 with: qemu-img-6.2.0-11.el9.x86_64 libvirt-daemon-8.0.0-5.el9.x86_64 many thanks, L.

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Network interface element not working
by Ian Pilcher 05 Apr '22

05 Apr '22

I've added an interface element to a libvirt network, but it isn't working. The interface is not being added to the bridge, even after the system is rebooted. # virsh net-dumpxml ocp4-net <network> <name>ocp4-net</name> <uuid>b5852945-9889-4d22-ba61-879125316cec</uuid> <forward dev='eno2' mode='nat'> <nat> <port start='1024' end='65535'/> </nat> <interface dev='eno2'/> </forward> <bridge name='virbr-ocp4' stp='on' delay='0'/> <mac address='52:54:00:99:99:99'/> <ip address='192.168.123.1' netmask='255.255.255.0'> </ip> </network> # brctl show bridge name bridge id STP enabled interfaces virbr-ocp4 8000.525400999999 yes vnet0 virbr0 8000.525400a7ce7f yes virbr1 8000.52540051eb1f yes vnet1 # rpm -q libvirt libvirt-8.0.0-2.module_el8.6.0+1087+b42c8331.x86_64 Any ideas? -- ======================================================================== Google Where SkyNet meets Idiocracy ========================================================================

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Guest restarting after issuing a shutdown
by jonetsu 04 Apr '22

04 Apr '22

Hello everyone, I create kvm machines using a bash script. As expected from the script, the resulting virtual machines are performing as they should be. The aspect that I do not get is at the end of the creation, when issuing a 'shutdown now' in the guest and expecting that it will shut down. Instead it does shut down, but it is restarted immediately. The machines are created using an Arch Linux ISO file. The bash script runs in three stages, the first stage is on the host. virt-install will show the regular Arch Linux installation interface. The bash script is copied to the machine and its second stage is ran. After that a chroot is done and the third phase of the script is executed. When everything is configured and installed, here is what happens, commands are prefixed '%', text output is as is, and comments in '()' : guest : % shutdown now host : Domain creation completed. host : Restarting guest. (virt-manager shows the guest restarting) host : % virsh shutdown test02 (nothing happens, guest still shown) host : % virsh destroy test02 host : Domain test02 destroyed (guest display closes) What I'm expecting is that, when the 'shutdown now' is issued in the guest, that it will effectively terminate the guest, 'virsh list --all' will not show it running, and there'll be no need to issue a 'virsh destroy'. I have no clue at the moment why virsh is restarting the guest after a shutdown was issued in the guest. Looks like virsh did not get that it should terminate the guest and considers the shutdown as an error. Here are the versions involved : host : xubuntu 20.04 LTS libvirt 6.0.0-0ubuntu8.15 virt-manager 1:2.2.1-3ubuntu2.1 guest : archlinux 2022.03.01-x86_64 Thanks !

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