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I have 32bit windows 2k3r3 guest (terminal server) with 4GB guest ram and swapping.

I created separate disk image for guest swapping and user's temp dirs.

I have enouth RAM in host system and want to save disk IO by moving this image to tmpfs, but guest doesn't start with this error message:

 qemu-kvm: -drive file=/mnt/tmpfs/vh1-tmp.qcow2,if=none,id=drive-ide0-1-1,format=qcow2,cache=none: could not open disk imag│ 4098 qemu 20 0 4949M 4146M 5496 S 28.5 17.2 1h00:31 /usr/bin/qemu-kvm -name vh1 -S -M pc-1.3 -cpu kvm64 -enable- e /mnt/tmpfs/vh1-tmp.qcow2: Invalid argument

Host system:

 #uname -a Linux srv-vh1.su.local 3.7.10-1.16-default #1 SMP Fri May 31 20:21:23 UTC 2013 x86_64 x86_64 x86_64 GNU/Linux srv-vh1:/mnt/tmpfs # virsh version Compiled against library: libvirt 1.0.2 Using library: libvirt 1.0.2 Using API: QEMU 1.0.2 Running hypervisor: QEMU 1.3.0 srv-vh1:/mnt/tmpfs # free total used free shared buffers cached Mem: 24627548 5084724 19542824 0 60640 138792 -/+ buffers/cache: 4885292 19742256 Swap: 8384444 0 8384444 srv-vh1:/mnt/tmpfs # cat /etc/mtab | grep tmpfs devtmpfs /dev devtmpfs rw,relatime,size=12296608k,nr_inodes=3074152,mode=755 0 0 tmpfs /dev/shm tmpfs rw,relatime 0 0 tmpfs /run tmpfs rw,nosuid,nodev,relatime,mode=755 0 0 tmpfs /sys/fs/cgroup tmpfs rw,nosuid,nodev,noexec,mode=755 0 0 tmpfs /mnt/tmpfs tmpfs rw,relatime 0 0 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ tmpfs /tmp tmpfs rw,relatime 0 0 tmpfs /var/lock tmpfs rw,nosuid,nodev,relatime,mode=755 0 0 tmpfs /var/run tmpfs rw,nosuid,nodev,relatime,mode=755 0 0 srv-vh1:/mnt/tmpfs # df Filesystem 1K-blocks used Avaible Used% Mountpoint devtmpfs 12296608 68 12296540 1% /dev tmpfs 12313772 0 12313772 0% /dev/shm tmpfs 12313772 6772 12307000 1% /run /dev/md1 454131992 218835836 212227596 51% / tmpfs 12313772 0 12313772 0% /sys/fs/cgroup tmpfs 12313772 192 12313580 1% /mnt/tmpfs ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ tmpfs 12313772 20 12313752 1% /tmp tmpfs 12313772 6772 12307000 1% /var/lock tmpfs 12313772 6772 12307000 1% /var/run srv-vh1:/mnt/tmpfs # virsh pool-info tmpfs Name: tmpfs UUID: 6287028a-9faf-f762-20de-d36d63657be3 Status: working Persistent: yes Autostart: yes Capacity: 11,74 GiB Выделение: 0,00 Avaible: 11,74 GiB srv-vh1:/mnt/tmpfs # ls -la total 196 drwxrwxrwt 2 root root 60 сен 9 11:42 . drwxrwxr-x 4 qemu qemu 4096 сен 8 19:39 .. -rw-rw-rw- 1 root root 197120 сен 9 11:42 tserver-tmp.qcow2

What am i doing wrong ?

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  • Check the logs, of course. Commented Sep 9, 2013 at 13:39
  • my error message is from libvirt log, this is the only message i have got on this Commented Sep 9, 2013 at 14:31

3 Answers 3

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Apparently, if you set cache=NONE for a disk image file on any host filesystem that does not support Direct IO, Virt-Manager will currently give a not very helpful error message saying "Something something... Invalid Argument" and refuse to start the Guest VM.

One example of such a filesystem --that does not support Direct IO-- is the tmpfs. Another such filesystem where maybe you can optionally disable Direct IO is GlusterFS (which I had not heard of before I also hit the same problem as Sergei did and was researching the error.) For tmpfs, not supporting Direct IO seems to be a technical limitation or design conflict at present and I don't know whether it will/can be rectified in future.

In my case, I had put a 3.6 GB Ramdisk for a Win7Pro Guest VM running on CentOS7 and had set cache=NONE for the ramdisk in Virt-Manager. The other options that the tmpfs img used were virtio and raw. The VM would refuse to start with the same/similar error saying "... Invalid Argument".

For technical details and notes directly from the Redhat Engineer and Developer that coded the patch for the feature cache=NONE & maintains (?) Virt-Manager (Daniel Berrange), see the discussion at the following link:

https://bugs.launchpad.net/nova/+bug/959637

To quote Daniel from the URL above: " > could not open disk image /mnt/vmstore/instances/instance-0000001a/disk: Invalid argument

probably means that the filesystem does not support Direct IO. AFAIK, all filesystems except tmpfs should support this.."

Also, Daniel continues: "- So what we'll want todo then, is to [....] do a check whether the storage volume supports Direct IO. If it does, then use cache=none, otherwise fallback to cache=writethrough which does not use Direct I/O, but is still crash safe."

In my case, I was able to verify that setting cache=NONE for the tmpfs img file was NOT starting the VM and showed the error as discussed. It WAS able to successfully start the VM if cache was set to either "Default" or explicitly "Write-through". There is no sense in going with "Write-back" since this filesystem both expendable and was anyways entirely in RAM, so obviously I did not use Write-back.

Hope that helps!

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Why not just give the guest more ram so it doesn't need to swap?

srv-vh1:/mnt/tmpfs # ls -la total 196 drwxrwxrwt 2 root root 60 сен 9 11:42 . drwxrwxr-x 4 qemu qemu 4096 сен 8 19:39 .. -rw-rw-rw- 1 root root 197120 сен 9 11:42 tserver-tmp.qcow2

I notice you have the folder owner set to qemu:qemu, if you are running qemu as a different user then you might want to change the owner of the image file from root to qemu

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  • i try that, with no result. And any operation with VirtManager chowns it back to root. What's why 666. Commented Sep 9, 2013 at 10:57
  • "Why not just give the guest more ram so it doesn't need to swap?" - 32bit guest, already have 4Gigs of RAM. Commented Sep 9, 2013 at 12:33
  • You could enable Physical Address Extension (PAE) in the guest OS, which will allow the guest to address more than 4GB of ram Commented Sep 9, 2013 at 22:02
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It's a bug. tmpfs doesn't support cache=none.

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