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SQL Server Tips by Burleson |
Indexes and Fillfactor
Some never look at or consider fillfactor when creating indexes for
a table, which may be just fine in some cases. In other situations,
pausing to consider the fillfactor effect for indexes is time well
spent.
As a quick review, the fillfactor effect specifies the percentage of
space filled on index data pages when an index is initially created.
The default fillfactor setting of zero, which can be altered at the
global server level through a configuration option, will cause an
index to be almost filled to capacity, with only a small amount of
space being left at the upper level region of the index. A 100%
setting completely fills each index page.
One important thing to remember is that this amount is not adhered
to after the index is first built. An index can be rebuilt and the
original fillfactor setting re-instituted with the variety of DBCC
index rebuild or ALTER INDEX (SQL Server 2005) commands.
So, what exactly are the considerations with fillfactor? Higher
fillfactor settings should result in less index pages which in turn
should result in fewer pages read during scan operations. As been
mentioned many times already, less I/O generally equates to better
performance.
However, high fillfactor settings can also result in page splits for
clustered indexes when SQL Server enforces the sort order of the
clustered index during INSERT or UPDATE actions. This happens
because SQL Server does not have room on an index page for the
requested change, so it has to split the page to perform the
modification. This can result in performance degradation and can be
confirmed by carefully watching the page splits counter, available
from the page_splits query below:
* page_splits.sql
-- Copyright © 2005 by Rampant TechPress
-- This script is free for non-commercial purposes
-- with no warranties. Use at your own risk.
--
-- To license this script for a commercial purpose,
-- contact rtp AT rampant.cc
-- *************************************************
select
cntr_value
from
master.dbo.sysperfinfo
where
counter_name = 'Page Splits/sec' and
object_name like '%Access methods%'
What rules of thumb should be followed with respect to fillfactor?
If there are tables present that are primarily read only, a
fillfactor setting of 100 should be used to reduce the number of
produced index pages.
If, however, there are tables present with high rates of INSERT,
UPDATE and DELETE activity, lower fillfactor settings of 50-60%
should be used. This will need to be coupled with periodic index
rebuilds that will re-establish the fillfactor setting to keep DML
running smooth through the indexes.
A mixed environment can work well with fillfactor settings in the
neighborhood of 75%. One last piece of advice: for small indexes
that have few pages, time should not be wasted worrying about
fillfactor as it will not be capable of impacting the database’s
performance for the worse.
The above book excerpt is from:
High-Performance SQL Server DBA
Tuning & Optimization Secrets
ISBN:
0-9761573-6-5
Robin Schumacher
http://www.rampant-books.com/book_2005_2_sql_server_dba.htm |
