Sun Logical Domains Open Door To Powerful UNIX Virtualization

Sun announced a new virtualization function for its Sun Fire T1000 and T2000 servers called Logical Domains (LDOMs). LDOMs are partitions that enable UltraSPARC-based servers to run multiple instances of operating systems simultaneously, whereby each instance can consume as little as a single processing thread. LDOMs complement Sun’s existing virtualization functions, including Dynamic Domains (i.e. hard partitions) and Solaris Containers (i.e. OS virtualization). Although each of these technologies provide similar capabilities as LDOMs, they have varying levels of granularity and different mechanisms for maintaining isolation between OS instances.

Indeed, LDOMs overcome some of the limitations of both Dynamic Domains and Solaris Containers. For example, LDOMs can be configured to use a fraction of CoolThreads processors, enabling far more precise utilization of server resources than Dynamic Domains, which require partitions to be configured in multiples of four processors. At the same time, LDOMs are more suited for general-purpose workloads than Solaris Containers, because they actually run their own instance of an operating system. Users may also have greater trust in the isolation provided by the firmware-based hypervisor in LDOMs than in the pure software implementation of Solaris Containers (see design presentation - PDF) .

From a purely functional standpoint, Sun's introduction of LDOMs starts to close a competitive gap with its UNIX competitors, including HP and IBM, each of which have UNIX virtualization functions that required fewer compromises than Sun’s earlier offerings. For T1000/T2000 users, LDOM’s ability to run multiple customized copies of Solaris 10 may facilitate certain types of workload consolidation more flexibly than Solaris Containers, and their firmware-based design may also offer a more reliable platform for hosting Linux workloads compared to the pure software-based implementations of industry-standard virtualization products such as VMware or Xen.

However, the real value of LDOMs, hosting critical UNIX applications with fluctuating workloads more economically by configuring hosts for average usage rather than over-provisioning for spikes, may await implementation on Sun’s next-generation high-end server systems based on the Advanced Product Line (APL) processor. Neither Sun nor partner Fujitsu have promised LDOMs on APL-based systems. But on these platforms, which will have the ability to support true SMP implementations rather than the thread-based multiprocessing of CoolThreads, LDOMs will be suitable for efficiently scaling typical UNIX workloads, rather than only new applications with multithreaded designs.