Understanding Memory - SQL Server 2012 Internals and Troubleshooting - page 54

The i rst part of this chapter explains the fundamentals of how Windows manages memory and
explains the difference and relationship between physical and virtual memory. The second part
focuses on SQL Server's internal memory structures, how they are managed, and how you can
break down their usage for effective troubleshooting. The chapter concludes with a look at different
memory coni guration options for SQL Server, helping you to understand what they do and how to
decide on an appropriate coni guration.
THE 32-BIT AND 64-BIT ARCHITECTURES
Support for 32-bit architectures (or more specii cally x86) continues with
SQL Server 2012, although there is one major change: removal of the Address
Windowing Extensions (AWE) feature, which allows 32-bit processes to access more
than 4GB of RAM.
The removal of AWE from SQL Server 2012 is really the i nal nail in the cofi n
for 32-bit SQL Server, so we anticipate that nearly all installations will be 64-bit.
Additionally, Windows Server 2012 is 64-bit only. For this reason, the rest of the
chapter focuses on and refers to only 64-bit SQL Server unless explicitly stated.
If you still have a requirement for 32-bit, the / 3GB tuning parameter is still supported
in SQL Server 2012 and is covered in detail in the “Tuning 32-Bit Systems”
section in Chapter 2 of the previous edition of this topic, Professional SQL Server
2008 Internals and Troubleshooting (Bolton et al., Wrox, 2010) .
PHYSICAL AND VIRTUAL MEMORY
This section covers topics — often considered to be outside the scope of a database professional —
that are fundamental to the way that Windows manages memory and the applications running on
it, including SQL Server. Understanding this information is a great differentiator among database
professionals and it will give you the right foundation of knowledge to understand how all
applications work with Windows.
Physical Memory
When the term physical memory is used, it's usually in relation to RAM (random access memory),
but it actually also includes the system page i le (explained later in the chapter). RAM is also
referred to as primary storage, main memory, or system memory because it's directly addressable by
the CPU. It is regarded as the fastest type of storage you can use, but it's volatile, meaning you lose
what was stored when you reboot the computer. It's also expensive and limited in capacity compared
to nonvolatile storage such as a hard disk.
For example, Windows Server 2012 supports up to 4TB of RAM, but buying a server with that
much memory will cost you millions of U.S. dollars, whereas it's possible to buy a single 4TB
hard disk for a few hundred dollars. Combine a few of those and you can have tens of TBs of very