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As you learned in chapter 5, configuring memory maximums in a multi-instance cluster is important in ensuring stability during failover situations. You must ensure that the total maximum memory values across all instances in the cluster is less than the total available memory on any one cluster node that the instances may end up running on during node outage. Setting the maximum memory values in such a manner is important to ensure adequate and consistent performance during failover scenarios. AMOUNT OF MEMORY TO LEAVE WINDOWS One of the important memory configuration considerations, particularly for 32-bit AWE systems and 64-bit systems that lock pages in memory, is the amount of memory to leave Windows. For example, in a dedicated SQL Server system with 32GB of memory, we ll obviously want to give SQL Server as much memory as possible, but how much can be safely allocated Put another way, what should the maximum memory value be set to Let s consider what other possible components require RAM: Windows Drivers for host bus adapter (HBA) cards, tape drives, and so forth

Antivirus software Backup software Microsoft Operations Manager (MOM) agents, or other monitoring software As shown in figure 7.4, in addition to the above non SQL Server components, there are a number of SQL Server objects that use memory from outside of the buffer pool that is, the memory area defined by the maximum memory setting. Memory for objects such as linked servers, extended stored SQL Server buffer pool (8K memory allocations) procedures, and object linking and embedding (OLE) automation objects is allocated from an Data cache area commonly called the MemToLeave area.

Worker threads, linked servers, OLE, etc., Multipage allocator (> 8K memory allocations)

Figure 7.4 The SQL Server buffer pool, as defined by the Max Server Memory setting, must share the server s physical memory with other memory consumers such as Windows and MemToLeave.

As you can see, even on a dedicated server, there s a potentially large number of components vying for memory access, all of which comes from outside the buffer pool, so leaving enough memory is crucial for a smooth-running server. The basic rule of thumb when allocating maximum memory is that the total of each instance s maximum memory values should be at least 2GB less than the total physical memory installed in the server; however, for some systems, leaving 2GB of memory may not be enough. For systems with 32GB of memory, a commonly used value for Max Server Memory (totaled across all installed instances) is 28GB, leaving 4GB for the operating system and other components that we discussed earlier. Given the wide variety of possible system configuration and usage, it s not possible to come up a single best figure for the Max Server Memory value. Determining the best maximum value for an environment is another example of the importance of a load-testing environment configured identically to production, and an accurate test plan that loads the system as per the expected production load. Load testing in such an environment will satisfy expectations of likely production performance, and offers the opportunity to test various settings and observe the resulting performance. One of the great things about SQL Server, particularly the recent releases, is its selftuning nature. Its default settings, together with its ability to sense and adjust, make the job of a DBA somewhat easier. In the next section, we ll see how these attributes apply to CPU configuration.

When an instance of SQL server starts, it s created as an operating system process. Unlike a simple application that performs a series of serial tasks on a single CPU, SQL Server is a complex application that must support hundreds or even thousands of simultaneous requests. In order to do this, the SQL Server process creates threads. A multithreaded operating system such as Windows allows applications like SQL Server to create multiple threads in order to maximize CPU efficiency and application