Understanding SQL Server Memory Internals

Like all database servers, Microsoft SQL Server thrives on memory. Each instance of Microsoft SQL Server has its own memory address space. The SQL Server address space (Virtual memory allocation by Windows) consists of two main components: executable code and memory pool. Let's examine each of these components in more details.

SQL Server executable code

Executable code is basically SQL Server's own EXEs and DDLs that are loaded into the address space. The following is breakdown of the various components:

• SQL Server Engine
• Open Data Services
• Server Net-Libraries
• Distributed Queries
• Extended Stored Procedures (DLLs)
• OLE Automation Stored Procedures

SQL Server memory pool

The memory pool is the unit of memory that an instance of SQL Server uses to create and manage data structures relating to client requests. The following data structures are allocated in the memory pool of an instance of SQL Server:

Buffer Cache

This is the pool of memory pages into which data pages are read. An important indicator of the performance of the buffer cache is the Buffer Cache Hit Ratio performance counter. It indicates the percentage of data pages found in the buffer cache as opposed to disk. A value of 95% indicates that pages were found in memory 95% of the time. The other 5% required physical disk access. A consistent value below 90% indicates that more physical memory is needed on the server.

Procedure Cache

This is the pool of memory pages containing the execution plans for all Transact-SQL statements currently executing in the instance. An important indicator of the performance of the procedure cache is the Procedure Cache Hit Ratio performance counter. It indicates the percentage of execution plan pages found in memory as opposed to disk.

Log Caches

This is the pool of memory used to read and write log pages. Each log has a set of cache pages. The log caches are managed separately from the buffer cache to reduce the synchronization between log and data buffers.

Connection Context

Each connection has a set of data structures that record the current state of the connection. These data structures hold items such as parameter values for stored procedures, cursor positioning information, and tables currently being referenced.

System-level Data Structures

These are data structures that hold data global to the instance, such as database descriptors and the lock table.

The buffer cache, procedure cache, and log caches are the only memory elements whose size is controlled by SQL Server.

A very important aspect to watch for is whether SQL Server is using the maximum memory available on the system (assuming the system is dedicated to SQL Server). A system with a fully utilized memory may be prone to performance bottlenecks when competition for resources increases. Prepared Transact-SQL statements, for example, may suffer when the procedure cache is unable to expand due to fully utilized buffer caches.