SQL Server 2008 Hot Add CPU

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SQL Server 2005 introduced the idea of hot add memory and SQL Server 2008 now provides hot add CPU. Hot add CPU enables you to scale a database on demand by adding CPU resources without any downtime.

To utilize this cool feature, Hot Add CPU, your server configuration must meet the following requirements before you can use the hot add CPU feature:

  • The 64-bit edition of SQL Server 2008 Enterprise.
  • Windows Server® 2008 for Itanium-Based Systems Enterprise or the 64-bit edition of Windows Server 2008 Datacenter.
  • Hardware that supports CPU additions, or virtualization software.

What’s an affinity mask? In a nutshell, it’s a bitmap of all the CPUs on the machine that specifies which ones are available for general SQL Server Engine use, and which ones are available only for SQL Server I/O. There are two kinds of masks, a regular affinity mask, and an affinity I/O mask. The regular affinity mask, if all zeroes, says that Windows decides who get’s what CPU when. If the affinity mask is non-zero, then there’s a bit per CPU. If it’s set to 1 then SQL Server can use the CPU. If a bit is set to 1 in the affinity I/O mask, then the CPU can only be used for I/O. A bit cannot be set in both masks. Now – an affinity mask is 32-bits wide, so if you have more than 32 CPUs, you need to use two more affinity masks, called affinity64 and affinity64 I/O. These do the same thing but for CPUs 32-63 on the machine. – From Paul S. Randal Blog


* SQL Server does not automatically use the new CPU. To configure the server, you must run a RECONFIGURE command

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