Memory DR in Oracle VM Server for SPARC 2.0

One of the major new features in Oracle VM Server for SPARC 2.0 (in conjunction with Solaris 10 9/10 aka Solaris 10 Update 9) is the ability to dynamically add or remove memory from a running domain - so-called memory dynamic reconfiguration (or memory DR for short). There are a few important caveats to keep in mind when using the memory DR feature:
  • Requests to remove sizable amounts of memory can take a considerable amount of time
  • It is not guaranteed to successfully remove all the memory requested (a smaller amount can end up being removed)
  • Removing memory that was in use at boot time results in the "leaking" of the memory that was used to hold the mappings for the removed memory, making that (much smaller but not insignificant amount of) memory unavailable until the domain is rebooted

While these restrictions implicitly connote a set of reasonable recommendations (e.g. avoid using memory DR to remove a significant portion of a domain's memory), there is a particular use-case that's especially problematic: shrinking the factory-default control domain in advance of creating guest domains. We strongly recommend against relying on memory DR for this purpose. Although it is tempting to try because it would eliminate the need to reboot the control domain to complete the process, the risks outweigh the benefits.

This issue and a procedure to avoid it are described in the Initial Configuration of the Control Domain section of the Oracle VM Server for SPARC 2.0 Administration Guide. I recommend folks read it in its entirety. In brief, it involves the use of a new CLI operation introduced in 2.0 to explicitly place the control domain into a delayed reconfiguration mode:

# ldm start-reconf primary
Note that if you don't explicitly execute this command before you attempt to resize the memory of the factory-default control domain (and you're running 2.0 with Solaris 10 Update 9 or later), memory DR will be invoked by default, potentially leading to the issues listed above. Caveat Emptor!
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I work on the Oracle VM Server for SPARC (nee LDoms) team.

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