Comparison of Raw,cow,qcow,qcow2 images

in theLinux, there are many ways to choose a virtual machine, such asVMware for Linux,Virtual Box, there areQemu, in the past, usingQemuthe people are not much, mainly used up some trouble, but now withOpenstack's rise,Qemualso got a lot of development now inFedoraunder UseQemu+Kvmperformance is still good, if you addSpice, it's even more nice. But today is still the main story.Qemuuse several image formats below!

1. Raw

raw format is the simplest and nothing, so called Raw format. Not even a header file is a file that reads and writes directly to the virtual machine. raw< Span lang= "ZH-CN" does not support dynamic growth space, you must specify the space size from the beginning. So the disk space is quite expensive. However, for file systems that support sparse files (such as ext4 ext4< The files created by default in span lang= "ZH-CN" are sparse files, so don't do any extra work. With

Du-sh file name

You can view the actual size of the file. In other words, no matter how large the disk space is, running the following instruction does not have any problems:

Qemu-img create-f Raw Test.img 10000G

The raw image format is one of the best I/O performance formats for virtual machines , and when used , they are referenced to raw and the better the performance is to Raw . However , raw does not have any other features. For sparse files, there is no advantage to the image of a runtime allocation space such as Qcow.

2. Cow

The Cow format is as simple as raw and allocates all the space when created, but Cow has a bitmap table that records which sectors are currently being used, so cow can use an incremental image. This means that you can take an external snapshot of it. But Cow also has no other function, its characteristic is simple.

3. Qcow

Qcowin theCowadded the ability to dynamically increase file size, and support encryption, compression. Qcowthrough2level Index table to manage the spatial allocation of the entire image, where the second-level index uses memoryCachetechnology that needs to find action, which leads to loss of performance. Qcownow basically not, on the one hand its optimization and function is notQcow2Well, on the other hand, read and write performance is notCowand theRawgood.

4. Qcow2

Qcow2 is a combination of various technologies of the Super image format, support internal snapshots, encryption, compression and a series of functions, access performance is also increasing. But the problem with Qcow2 is that it's too bloated to have all the functions in one.

Comparison of Raw,cow,qcow,qcow2 images