[PVE-User] Image format of disc

Daniel Pittman daniel at rimspace.net
Thu May 20 15:15:49 CEST 2010

"Erik van Ast" <Erik.van.Ast at suzohapp.nl> writes:

> Can someone tell me what kind of image format we should use when creating
> new VM's?  We have 2 VM's running at the moment. One is using VMDK because I
> copied it from a VMWARE ESX server, and the other is in RAW format because I
> also created a new VM and RAS is the default setting. Which one is
> considered best and most reliable?

Heh.  This is relevant to me next question, so let me answer this bit on the
way through:

Generally, raw is the best performing option, and is preferable if you can
allocate a block of disk storage (eg: an LV) to the system.

QCOW2 is the best choice if you want a file sitting on top of a file-system,
for some reason.

Meanwhile, my question was specifically about performance: ideally, I would
like to have my KVM systems configured with raw storage backed by an LV, and
with the 'cache=none' option passed to the kvm binary.

That allows the kernel to bypass copies of the data, and the host cache, which
given that I have allocated almost all memory to the KVM guest, is what I
actually want to happen.

Is it possible to tune these fine details along with the gross storage format

✣ Daniel Pittman            ✉ daniel at rimspace.net            ☎ +61 401 155 707
               ♽ made with 100 percent post-consumer electrons

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