1.4.2. Para-virtualized drivers
Para-virtualized drivers are device drivers that increase the I/O performance of virtualized guests.
Para-virtualized drivers decrease I/O latency and increase I/O throughput to near bare-metal levels. It is recommended to use the para-virtualized drivers for virtualized guests running I/O intensive applications.
The para-virtualized drivers must be installed on the guest operating system. By default, the para-virtualized drivers are included in Red Hat Enterprise Linux 4.7 and newer, Red Hat Enterprise Linux 5.4 and newer and Red Hat Enterprise Linux 6.0 and newer. The para-virtualized drivers must be manually installed on Windows guests. For more information on using the para-virtualized drivers refer to
Chapter 11, KVM Para-virtualized Drivers.
The para-virtualized network driver is a Red Hat branded virtual network device. The para-virtualized network driver can be used as the driver for existing network devices or new network devices for virtualized guests.
The para-virtualized block driver is a driver for all storage devices supported by the hypervisor attached to the virtualized guest (except for floppy disk drives, which must be emulated).
Guests using the Time Stamp Counter (TSC) as a clock source may suffer timing issues.
KVM works around hosts that do not have a constant Time Stamp Counter by providing guests with a para-virtualized clock.
The para-virtualized serial driver (virtio-serial) is a bytestream-oriented, character stream driver. The para-virtualized serial driver provides a simple communication interface between the host's user space and the guest's user space where networking is not available or unusable.
The balloon driver allows guests to express to the hypervisor how much memory they require. The balloon driver allows the host to efficiently allocate memory to the guest and allow free memory to be allocated to other guests and processes.
Guests using the balloon driver can mark sections of the guest's RAM as not in use (balloon inflation). The hypervisor can free the memory and use the memory for other host processes or other guests on that host.
When the guest requires the freed memory again, the hypervisor can reallocate RAM to the guest (balloon deflation).
