Tuning SQL Server Memory in 2 Minutes

,

Introduction

You cannot

do a proper tuning job on SQL server in two minutes, but you can get a good

idea of how well a server is set up. This article will show you how to quickly

evaluate the efficiency of memory usage on your server.

 

The script

I use works both on SQL 6.5 and 7, though the results you get and the actions

you can take differ for each version. Some of the commands are in SQL 7 for

backward compatibility only, and MS recommend you use Performance Monitor

instead, but I still like to use these commands to get a quick idea of what is

happening.

 

Here is the

script:

 

dbcc traceon(3604)

go

dbcc sqlperf(lrustats)

go

dbcc bufcount(1)

go

-- dbcc memusage – Comment

out for SQL Server 7

go

dbcc proccache

go

dbcc traceoff(3604)

 

 

dbcc

traceon simply

turns on the output for some of the subsequent commands – without this you will

not see all the results you need. The Traceoff command at the end turns this

feature off again.

 

dbcc

sqlperf(lrustats) outputs

details of the cache performance. The most interesting figures are the “cache

hit ratio” – which should be near to 100% as possible, and “cache flushes” –

which tells you the number of times that data in cache has been paged out to

disk to make room for other data. In an ideal world this would be zero, but if

your database sizes add up to more memory than you have physical RAM allocated

to SQL Server then this will not usually be the case.

 

If these

figures do not look so good, the first place to look is the “Sp_configure

memory” command to see if SQL Server has enough memory allocated to it. This is

particularly important with 6.5, which has a very low default setting, but less

likely to be the cause of problems with SQL 7, which manages it’s memory

automatically. See the “Causes of cache confusion” at the bottom of this

article for other possible causes.

 

How much

memory you allocate to SQL Server 6.5 depends on what else your server is doing

as well as the total available memory. This Microsoft

article contains guidelines and example memory allocation settings.

 

dbcc

bufcount(1) SQL 7

users can skip this one as it’s configured automatically. SQL 6.5 users should

look for a line like “The Average Chain Size is: 2.922601” in the output. This

indicates the efficiency of the cache indexing structure – values between 2 and

4 are OK with 3 being the optimum in SQL 6.5. The adjuster for this is

“sp_configure hash buckets”, and because this is poorly documented in SQL 6.5

Books Online many sites still have the default setting - Read

more about this setting at the Microsoft support site.

 

dbcc memusage details the largest tables

currently in your cache. Ideally these should be pretty stable, so make a note

of what objects are in cache, and the space they take up, over a few readings.

SQL 6.5 will also detail the largest Stored Procedures in your Procedure Cache

- more on this below.

SQL

7 users be warned – this article says not to run the dbcc memusage

command on SQL 7.  I have never had

problems with it myself, but I would advise you not to use it on production

servers.

 

dbcc

proccache gives

more succinct details about the Procedure cache usage. Again SQL 7 handles this

pretty well on it’s own, but SQL 6.5 usually needs some help. This is because

SQL 6.5 by default allocates 30% of it’s total cache memory to handling

(mostly) stored procedures, while the rest is give over to data cache. With

large memory configurations this can result in a lot of wasted space in

Procedure Cache that could be better used more effectively as Data Cache. The

adjuster for this is “sp_configure procedure cache”.

Take

care when reducing this setting – setting procedure cache too low can kill your server. Reduce it

slowly and carefully, and never ever set it to zero. Here is a Microsoft

article on the subject.

Causes of cache confusion

The most

obvious reasons for inefficient cache usage are lack of available RAM and poor

memory configuration, but before you rush out and buy some new DIMMS think

about some other possible causes.

 

  • Missing Indexes can cause

    unnecessary table scans, which clear other data out of cache. SQL 7 users

    can consider using SQL Profiler and the Index

    Wizard to spot table scans and improve index efficiency. Unfortunately

    SQL 6.5 users have to do this the hard way.

  • Poor database design and poor

    query design can lead to table scanning, excessive disk IO and inefficient

    memory usage.

  • Mixing databases with OLTP

    (short transaction) and OLAP (long report) characteristics on the same

    server can cause inefficient cache use and other problems

 

Here is a

good article

from Microsoft on the importance of good database and query design.

 

Finally,

remember that caching statistics are meaningless if your server has just

started. Cache efficiency increases over time, so let your server “settle down”

into it’s normal usage pattern before you try to measure efficiency.

 

Other memory

considerations

Cache is

one of the most important uses of RAM, but it by no means the only important

one, and SQL Server allocates memory to cache only after other requirements are

met.

 

It follows

that you can increase or decrease the amount of RAM available for cache by

adjusting the memory allocation for other aspects of SQL Server, particularly

with version 6.5, which allocates memory in a less dynamic way than later

versions.

 

Some

example settings you can change are Sort Pages (6.5 only) Index Create Memory

(7 only) Locks, Open Objects, Tempdb In RAM (6.5 only, and usually

not recommended) There are others too, and as you read more in Books Online

or http://support.microoft.com Be

warned though: an incorrect setting for any one these can kill your server, so

read up first, and adjust things slowly and carefully.

About the author

Neil Boyle is an independent SQL Server consultant working out of London,

England. Neil's free SQL Server guide is available on-line at http://www.impetus-sql.co.uk

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