Talk:QEMU

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Revision as of 20:57, 21 February 2014 by Lahwaacz (Talk | contribs) (keep the chronological order please)

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Linear RAID

When I was updating the article yesterday, I had tried to fit the section about linear raid (boot a VM from a partition by prepending a MBR to it) into the article better. But I'm not sure the technique described is the right one at all. It looks like it works, but wouldn't it be easier to install a bootloader directly to the partition (e.g. syslinux)? Then the VM could be booted directly from the partition simply by using it as its virtual disk. --Synchronicity (talk) 19:23, 9 May 2012 (UTC)

Creating bridge manually

I really don't know what to do with this section. I'd say it has been superseded by QEMU#Creating bridge using qemu-bridge-helper (available since qemu-1.1, we now have qemu-1.5) - or is someone still using this method? Perhaps link to https://en.wikibooks.org/wiki/QEMU/Networking#TAP_interfaces or http://wiki.qemu.org/Documentation/Networking/NAT is sufficient. What do you think? -- Lahwaacz (talk) 20:42, 22 July 2013 (UTC)

Actually, I've become a happy user of this method. I've written some scripts to easily create bridge interface, TAP interface, and combined with Xyne's excellent scripts to set up NAT and launch DHCP server, I have complete solution to easily manage multiple VMs on one (or even more) bridge.
My scripts are available on github: [1], [2], [3] but I won't probably integrate them into the wiki, I'l just leave a note when I do some more testing.
The thing is, what to do with the current content? Personally I think that links to [4], [5] and my scripts are sufficient (of course others are welcome). I'd also leave the note at the end to disable the firewall on the bridge, I find it extremely useful.
-- Lahwaacz (talk) 18:24, 5 September 2013 (UTC)

Preparing an (Arch) Linux guest

adding the virtio modules to use virtual devices doesn't appear to be necessary as of the VM I built this week(Jul 26, 2013). Kernel 3.9.9-1 Surlyjake (talk) 03:42, 27 July 2013 (UTC)

In newer kernel versions (I think since 3.4) the kernel modules are loaded automatically. I'd still keep the section as it is for now, linux-lts is still at 3.0. -- Lahwaacz (talk) 21:09, 20 August 2013 (UTC)
linux-lts is now on the 3.10 series.
-- Jstjohn (talk) 16:51, 23 September 2013 (UTC)

Starting QEMU virtual machines with Systemd

The custom systemd service script does not work. It always fails with Failed at step EXEC spawning /usr/bin/qemu-{type}: No such file or directory. To Fix this modify the ExecStart command
ExecStart=/usr/bin/sh -c "/usr/bin/qemu-${type} -name %i -nographic ${args}"
Also echo 'system_powerdown' | nc localhost 7101 kills the VM immediatly. To fix this change the stop script. It simply checks each second if the main process is still running.
ExecStop=/usr/bin/sh -c "${haltcmd} && while [[ `pidof qemu-${type} | grep $MAINPID` ]]; do sleep 1; done"

gnu-netcat does not work to connect to the monitor. You need to use openbsd-netcat. -- Ant32 (talk) 17:48, 5 September 2013 (UTC)

The first problem related to starting the service seems rather strange - didn't you have typo error in your local qemu@.service file (missing the dollar sign $ in ${type} )?
The second problem is valid, systemd kills the main process when the ExecStop command exits (see systemd.service(5)). If your workaround really works, it could be added to the wiki with a proper description.
-- Lahwaacz (talk) 21:17, 7 September 2013 (UTC)
Relevant thread on systemd-devel mailing list: [6] -- Lahwaacz (talk) 00:00, 15 September 2013 (UTC)

Kexec Hackery When Using a Real Partition

After banging my head against a wall long enough and figuring out what -kernel and -initrd were really calling, I put a note above the appropriate section and mentioned two ways to use the guest's images. (Otherwise, you'll have to worry if the host and guest images match.) The first -- mount the partition(s) -- is more appropriate for "low-volume-handling" of VMs. The second -- using kexec -- becomes more useful when you're juggling more than a few VMs.

I'm only mentioning this hack because (as of now) Kexec only mentions use for rebooting into another kernel, not switching out the kernel before the system is even up. This hack comes from from https://digitalocean.uservoice.com/forums/136585-digitalocean/suggestions/2814988-give-option-to-use-the-droplet-s-own-bootloader- which has two suggestions. The most recent, using systemd units by jkuan, doesn't work because jkuan tried to copy a .target file into a .service file and systemd wants ExecStart in a .service file. The second one, replacing /usr/bin/init by andrew_sparks, works for me on my Arch instance at DigitalOcean.

Adaptation from said post:

# pacman -S kexec-tools
# pacman -R systemd-sysvcompat
/tmp/init
#!/bin/sh

kexec --load /boot/vmlinuz-linux --initrd=/boot/initramfs-linux.img --append="root=/dev/sda init=/usr/lib/systemd/systemd" &&
mount -o ro,remount / && kexec -e
exec /usr/lib/systemd/systemd
# cd [/path/to/vm]/usr/bin
# mv init init.dist
# cp /tmp/init ./
# chmod 755 init

I'm leaving this on the Talk page as I haven't even tried it out in QEMU myself. Also, my eyes are about ready to pop out of my head, so I'm barring myself from figuring out the appropriate way to edit this in for the time being. BrainwreckedTech (talk) 21:23, 14 January 2014 (UTC)

Resizing image containing NTFS boot system

The following is simple and worked for me - can someone validate before I add to the wiki please:

One simple way to resize images containing an NTFS boot system is to convert to raw format and add blank space onto the end of the image. This leaves sector sizes and partition start locations unchanged. For example, to add 4GB to a qcow2 image, first shut down any VM's using the image, then:
# qemu-img convert -f qcow2 -O raw myimg.qcow2 myimg.raw
# truncate -s +4G myimg.raw # (this adds 4 GB of blank space at the end of the image)
# mv myimg.qcow2 myimg.qcow2.bak
# qemu-img convert -f raw -O qcow2 myimg.raw myimg.qcow2
Check that the permissions on the new image match the original. You should then be able to start the VM and resize the partitions on the image using your preferred tool. Note that from Windows Vista onwards, the built-in Disk Management utility lets you resize mounted partitions (right-click on Computer, Manage, Disk Management). Alternatively you can boot your VM from a bootable CD image which contains gparted or similar. Or if you know what you are doing, you can loop-mount the image and resize the partitions manually from the host OS.
Once you have verified that all is working, you can delete the .bak backup that you made along the way.

Thomas d j (talk) 20:56, 21 February 2014 (UTC)