What format to use for KVM Windows guests - RAW, QCOW2, VMDK

[Nachtfalke]

Hi everybody,

I was asking myself what would be the optimal format to chose for performance when using windows guests and KVM.
I have a 72GB SAS RAID-1 partition form ProxmoxVE3.0 and 500GB SATA RAID-1 mounted into a directory.

My intention is to do snapshots on the VMs and do backups to a fileserver automatically every week. I have maintenance windows so no need for live migration.

At the moment I am using qcow2 format and in the few tests I did it looks like I could do snapshots and backups with this format. So when reading in the wiki I found out that there may be performance leaks when using qcow2 and Microsoft SQL database. The wiki says to modify the SQL start parameter what I did and it says - better using RAW instead of qcow2.

So my questions are:
- Could I use RAW instead of qcow2 in my environment with my needs?
- Converting the VMs from qcow2 to RAW will probably work without problems using "qemu-img convert" command, right?
- Any other suggestion using the one format and not the other?
- I have some old WMDK VMs I need to use with proxmox. From performance view - any need to convert them from VMDK to qcow2 or VMDK to RAW?

Huh, more questions than I though

Thank you!

 

[mir]

- Could I use RAW instead of qcow2 in my environment with my needs?
- Converting the VMs from qcow2 to RAW will probably work without problems using "qemu-img convert" command, right?
- Any other suggestion using the one format and not the other?
- I have some old WMDK VMs I need to use with proxmox. From performance view - any need to convert them from VMDK to qcow2 or VMDK to RAW?

1) RAW gives more performance than qcow2
2) If you use latest stable pve then you are able to convert from qcow2 to raw using the command 'move disk'
3) qcow2 requires a file system so this is not available through LVM.
4) depending on your storage solution you might loose the ability to make snapshots if you use raw
5) vmdk should only be used as a last resort. qcow2 and raw are the preferred image format with kvm.

 

[Nachtfalke]

Thank you very much for your feedback. I used the following tutorial to add my second hard disk:
http://www.debiantutorials.com/add-a-second-hard-disk/

I added it as "Directory" on the PVE GUI and it is formatted as ext3. Would this allow me to make snapshots of RAW images?

 

[spirit]

Thank you very much for your feedback. I used the following tutorial to add my second hard disk:
http://www.debiantutorials.com/add-a-second-hard-disk/

I added it as "Directory" on the PVE GUI and it is formatted as ext3. Would this allow me to make snapshots of RAW images?

you can't do snapshot with raw file, only with qcow2. (But you can do live backup)

 

[Nachtfalke]

Thank you spirit,

will chose qcow2 as format and do live snapshots and offline backups. You posting and mir's helped me.

Have a nice weekend!

 

[acw-flo]

Hello,

I have a similar question.
I have a nfs storage on a zfs server for my kvm disks.
Normally, I use raw images, but now I want to ask which format is better if I need to have the ability to resize the disk.
I make daily snapshot on my zfs server, are there any problems when I use qcow2?

Is the performance really so much better with raw than qcow2?

cu Flo

 

[spirit]

Hello,

I have a similar question.
I have a nfs storage on a zfs server for my kvm disks.
Normally, I use raw images, but now I want to ask which format is better if I need to have the ability to resize the disk.
I make daily snapshot on my zfs server, are there any problems when I use qcow2?

Is the performance really so much better with raw than qcow2?

cu Flo

you can online resize a raw disk.
qcow2 benefit is mainly snapshots.

qcow2 can have write performance, mainly if you write small block. (maybe 20-30% slower)

 

[acw-flo]

you can online resize a raw disk.
qcow2 benefit is mainly snapshots.

qcow2 can have write performance, mainly if you write small block. (maybe 20-30% slower)

Hello spirit,

thank you for your quick reply!
Can I resize raw disk also if it is on a nfs share?

How can I do this?

 

[spirit]

Hello spirit,

thank you for your quick reply!
Can I resize raw disk also if it is on a nfs share?

How can I do this?

yes, sure,noproblem. just use resize disk button in gui