Block I/O tuning
Going back to basics – a virtual disk of a VM can be either a block device or an image file. For better VM performance, a block device-based virtual disk is preferred over an image file that resides on a remote filesystem such as NFS, GlusterFS, and so on. However, we cannot ignore that the file backend helps the virt admin to better manage guest disks and it is immensely helpful in some scenarios. From our experience, we have noticed most users make use of disk image files, especially when performance is not much of a concern. Keep in mind that the total number of virtual disks that can be attached to a VM has a limit. At the same time, there is no restriction on mixing and using block devices and files and using them as storage disks for the same guest.
A guest treats the virtual disk as its storage. When an application inside a guest OS writes data to the local storage of the guest system, it has to pass through a couple of layers. That said, this I/O...