INF: Estimating the Initial SQL Server Memory Setting (168697)

MORE INFORMATION

From the Systems Administration for Microsoft SQL Server 6.5 Student Workbook, the following data is provided for a start in allocating memory to SQL Server:

Physical RAM
on a computer
dedicated to
SQL Server       Memory dedicated to       sp_configure 'memory'
 (MB) {x}        SQL Server (MB) {y}       setting (in 2-KB pages)
---------------------------------------------------------------------------
   16                5.468                 2,800 (minimum, not recommended)
   24                  8                   4,096 (SQL Server 6.5 default)
   32                 16                   8,192
   48                 28                  14,336
   64                 40                  20,480
  128                100                  51,200
  256                216                 110,592
  512                464                 237,568
				

Using Excel Linear Regression capability, the last four values can be used to generate slope and intercepts for a best-fit line of the form y = mx+b. In this case, m = .9462 and b = (-22.09). For this purpose, .95 and (-22) will suffice, and the equation becomes y = (0.95)*x-22.

Thus, the appropriate SQL Server memory setting in MB is: (0.95)*(the amount of physical RAM on the computer, in MB) - 22.

Because the SQL Server memory setting is specified in 2-KB pages (for example, 2,048 bytes) and because 1 MB equals 1,048,576 bytes, the conversion factor from MB to 2-KB pages is (1,048,576/2,048) = 512 (2-KB pages/MB), the following equation delivers is the result:

The appropriate SQL Server sp_configure memory setting in 2-KB pages is: (((0.95)*(the amount of physical RAM on the computer, in MB) - 22)*512).

That is, y = (.95x-22)*512

Notes

1. This estimation is only for computers dedicated solely to the use of SQL Server (that is, the computer is not running as a primary or backup domain controller, nor is it running WINS, DHCP, IIS, file and print services, or anything else that takes memory other than Windows NT Server or Workstation and SQL Server). If the computer is running other services, you can typically size the largest amount of memory that all of the other services or applications will use (and add some extra) and increase the '22' number (that is, the intercept/offset/'b') in the equation by that amount of additional RAM (in MB) that is unavailable.
2. The data this is based on is from 64 to 512 MB of RAM. As with all statistics, you need to be aware of the statistical insignificance when going outside of the range of the raw dataset. Specifically, when the amount of available RAM is less than 64, ratio to nearest number in the table to determine what your value should be.
   
   For example, if your computer had 36 MB of RAM and was dedicated to SQL Server, you would make the following calculations:
   

         36 is 4 MB greater than 32, and because the gap in the table between
         32 and 48 is 16 MB, your 36 MB of RAM should call for a memory
         setting of 4/16=.25 of the gap between these to settings. That is,
         28 - 16 = 12 and (.25)*12 = 3, so for your 36-MB SQL Server computer
         dedicated to the server, a memory setting  of 16+3=19 MB is
         appropriate. Converting 19 MB to 2-KB pages implies an sp_configure
         memory setting of 19*512=9,728.
    
   						

   On computers with more than 512 MB of RAM, this equation often provides a good benchmark.
3. Although this data set was specifically listed for SQL Server 6.5, it seems to work well for SQL Server 6.0 too.
4. With an insufficiently sized NT Paging File, out-of-Virtual Memory symptoms may be seen. Check the Windows NT Resource Kit for more information on Virtual Memory.
5. SQL Server Memory configuration does not include the virtual address space needed for "Max Worker Threads" and "RA Worker Threads." With this in mind, ensure that you do not exceed the 2-GB Address space limit for SQL Server (3 GB for SQL Server Enterprise Edition) when estimating the initial memory setting. For additional information, click the article number below to view the article in the Microsoft Knowledge Base:

   110983 INF: Recommended SQL Server for WinNT Memory Configurations