Is online disk resize possible with KVM?

Solution 1:

I think you're stuck doing what you've mentioned if you want to do it without taking the machine down.

Why not just give the virtual machines LUNs right off the SAN and manage the space there? This works better if you want to use features like live migration anyhow.

KVM is based on QEMU so all it's image format support comes from that project. Here's a good how-to resize the various formats that Qemu/KVM support. But the Qemu forum would be a good place to ask this question if you don't get any solid answers here.

Another option which may not be ideal is to use really large qcow2 or other sparse image format for the drives. So you could give each machine a small drive for the OS and a large sparse image for data under LVM. This would at least keep the number of virtual drives/images that you're having to manage. But this thin provisioning could be a problem though if you do this to 1000 machines and everyone takes you up on the free space that they see.

XEN I believe has the same limitations currently.

Solution 2:

I know that's an old question but I've found it while googling for the solution and hope it may help someone else.

As for today it is possible to resize the hard drive on the machine. I've found a working way here:

https://bugzilla.redhat.com/show_bug.cgi?id=648594

The following steps must be performed:

1. Find out a file name and KVM device name of the hard drive you want to resize:

   root@vhstage02:/data# virsh dumpxml test | xpath -e /domain/devices/disk
   Found 2 nodes in stdin:
   -- NODE --
   <disk type="file" device="disk">
     <driver name="qemu" type="qcow2" />
     <source file="/data/test.img" />
     <backingStore />
     <target dev="vda" bus="virtio" />
     <alias name="virtio-disk0" />
     <address type="pci" domain="0x0000" bus="0x00" slot="0x04" function="0x0" />
   </disk>
   -- NODE --
   <disk type="file" device="cdrom">
     <driver name="qemu" type="raw" />
     <source file="/data/images/debian-8.2.0-amd64-netinst.iso" />
     <backingStore />
     <target dev="hda" bus="ide" />
     <readonly />
     <alias name="ide0-1-1" />
     <address type="drive" controller="0" bus="1" target="0" unit="1" />
   </disk>
   

The one interesting for us is disk. You should look for source and alias blocks. For me file name is test.img and alias name is virtio-disk0. To this name, you need to prepend drive- to get qemu drive name.

2. Now we actually resize the drive using qemu monitor:

   virsh qemu-monitor-command test block_resize  drive-virtio-disk0  100G --hmp
   

Note that filename was used without .img extension and drive- was added to disk alias. The 100G is the resulting size of the drive we want to have

3. Login to the machine and check that actual size was changed:

   root@test:~# fdisk -l
   
   Disk /dev/vda: 100 GiB, 107374182400 bytes, 209715200 sectors
   Units: sectors of 1 * 512 = 512 bytes
   Sector size (logical/physical): 512 bytes / 512 bytes
   I/O size (minimum/optimal): 512 bytes / 512 bytes
   Disklabel type: dos
   Disk identifier: 0x7e6e7f71
   
   Device     Boot  Start       End   Sectors  Size Id Type
   /dev/vda1  *      2048    499711    497664  243M 83 Linux
   /dev/vda2       501758 167770111 167268354 79.8G  5 Extended
   /dev/vda5       501760 167770111 167268352 79.8G 8e Linux LVM
   

That's it! Now you can either create new partitions or resize existing ones.