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RAW image format performance
Let's begin with some tests regarding the most basic disk image format – the RAW format. RAW images are very fast, but they miss a critical feature: the possibility to take real, fast snapshot of the virtual disk. So, they can be used only in situations were you not need a real snapshot support (or you have snapshotting capability at the filesystem level – but this is another story).
How the RAW format performs, and how caching affect the results?
As you can see, as long you stay away from write-through cache, RAW image have very high speed. Note that in RAW image with no caching or write-back policy, preallocation only have a small influence.
What about the much more feature-rich QCOW2 format?