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LDOM, short for Logical Domains, is a hypervisor-based virtualization technology developed by Sun for SPARC hardware to allow the use of virtual machines. It is similar to Xen on x86; however, it does not appear to have as many features as its counterpart yet (for instance, lack of live migration and limited support for dynamic reconfiguration of CPUs and RAM).
LDOMs currently only run on UltraSPARC T series processors. The technology does not allow oversubscription of processing power or RAM. As a result, the maximum of amount of LDOMs on a system are limited to the number of threads supported by the processors. For example, if the system has 2 processors, each with 4 cores and 8 threads/core, then 64 LDOMs including the primary (see below) can run on the system at any given point in time. Although this is the case, an LDOM best practices guide suggests that LDOMs be split along core boundaries for best performance.
On an LDOM system, there is a primary LDOM (analogous to the dom0 in Xen), and then there are secondary LDOMs (analogous to domUs in Xen). While the primary LDOM must run a recent version of Solaris 10, the secondary LDOM theoretically supports other UltraSPARC T2-compatible operating systems. Supported operating systems include Solaris 10, Red Hat Enterprise Linux, and Ubuntu Linux. We currently are trying to get a Gentoo LDOM going, but it is proceeding poorly at best.
Currently, the CSL has two servers that support LDOMs: