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virtual I/O by performing I/O to back-end devices, which are usually physical de-
vices. Virtual device characteristics are described in detail later in this chapter.
Guest domains have network and device drivers that communicate with I/O do-
mains through Logical Domain Channels (LDCs) provided by the hypervisor. The
addition of device drivers that use LDCs rather than physical I/O is one of the ar-
eas in which Solaris has been modified to run in a logical domain—, an example of
paravirtualization discussed in Chapter 1, “Introduction to Virtualization.” LDCs
provide communications channels between guests, and an API for enqueuing
and dequeuing messages that contain service requests and responses. Figure 3.2
shows the relationship between guest and service domains and the path of I/O
requests and responses.
Figure 3.2
Service Domains Provide Virtual I/O
Shared memory eliminates the overhead associated with copying buffers be-
tween domains. The processor's memory mapping unit (MMU) is used to map
shared buffers in physical memory into the address spaces of a guest and an I/O
domain. This strategy helps implement virtual I/O efficiently: Instead of copying
the results of a disk read from its own memory to a guest domain's memory, an I/O
domain can read directly into a buffer it shares with the guest. This highly secure
mechanism is controlled by hypervisor management of memory maps.
I/O domains are designed for high availability. Redundant I/O domains can be
set up so that system and guest operation can continue if a path fails, or if an I/O
domain fails or is taken down for service. Logical Domains provides virtual disk
multipathing, thereby ensuring that a virtual disk can remain accessible even
if a service domain fails. Domains can use IP network multipathing (IPMP) for
redundant network availability.
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