We
use the venerable dd command to create the file that will be used to store our image.
Doing this ensures that the file will be sparse, and space will be allocated only as
parts of the file are used, not up front when the file is created.
Although files are a simple and quick way of storing VMs, there are some
disadvantages of using files as virtual block devices:
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Storage
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They are not suitable for I/O-intensive uses, as they experience some
slowdown under heavy I/O workloads. The loopback block device used to
support file-backed VBDs in dom0 performs the I/O handling quite poorly.
Linux by default, supports a maximum of eight file-backed VBDs across all
domains. This limit can, however, be increased by utilizing the max_loop
kernel parameter if the CONFIG_BLK_DEV_LOOP command is compiled as a
module in the dom0 kernel.
NFS
Network File System (NFS) is a client/server system that enables users to access files
across a network and treat them as though they are in a local file directory. An NFS
server provides remote clients with access to its files by exporting the files that are
mounted by the remote client and made available to the operating system and
the user.
NFS is a great way to allow a system to share directories and files with other systems
over a network. It enables users to access files on remote systems almost as though
they were local files. Some of the advantages of NFS are as follows:
Disk space usage on local machines is reduced considerably by storing most
of the commonly used and accessed data on a single machine accessible to
others over the network.
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