[Linux-PowerEdge] KVM Disk Format Options

Mesh meshvic at gmail.com
Wed Feb 20 01:04:28 CST 2013

Good day,

We are implementing a multi-node system based on KVM that will have high
disk I/O and I'm doing some research so that I can advise whether to use
block devices (LUNs) for our VMs, or image files. I'm getting confusing
messages, depending on where I look, including some Dell white papers that
I could access.

Here is an example:

 Best practice: Use block devices for VM storage

A guest operating system that uses block devices for local mass storage
typically performs better than a guest operating system that uses disk
image files. The guest operating system that uses block devices achieves
lower-latency and higher throughput.

A guest operating system that uses block devices performs better because of
the number of software layers through which it passes. When an I/O request
is targeted to the local storage of a guest operating system, the I/O
request must pass through the file system and I/O subsystem of the guest
operating system. Then, qemu-kvm moves the I/O request from the guest
operating system and the hypervisor handles the I/O request. Next, the
hypervisor completes the I/O request in the same way the hypervisor
completes an I/O request for other processes running within the Linux
operating system.



We are implementing this system on five Dell blades and a Dell EMC storage
(iSCSI targets), which I assume will have some caching and other
technologies that will minimize the actual disk operations - I don't know
the inner-workings of EMC so some clarity here will also be appreciated.

The question I have is - while it is clear that for stand-alone servers
it's preferable to use block devices, are there any significant performance
benefits if LUNs are used instead of raw image files on centralized
storage?  What are the trade offs and where can I get some case
studies/white papers/publications/etc on this subject?

Your input will be highly appreciated.

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