It’s time for another SQL Server refresher today! This time we will discuss some storage basics, specifically Pages and Extents and how they relate to each other. There are a lot of resources out there discussing these storage units, but I’ve tried to put my own spin on things and aggregate as much data as I could find about them.
Naturally we are going to discuss pages first, since they are the most fundamental unit of storage for SQL Server. Pages store everything in the database and are only 8 KB small. Your entire disk I/O is performed at the page level.
Pages have three major components, a page header, records, and the offset array. Pages start with a 96 byte header which contains meta-data, like the page number, owner’s object id, and page type. Pages end with the offset array which is 36 bytes and has pointers to each new row stored in the page. These pointers are stored last to first, but that’s more easily explained in the picture. The offset array is essentially the index for the page. The middle of the page is the records, and consists of the remaining 8060 bytes containing stored data.
There are different types of pages, such as data, index, image, and a number of informational pages. Probably the most interesting type are overflow pages. If a row is greater than 8060 bytes, the data can be stored on overflow pages which are linked together. Overflow pages can store as much as 2GB in a single column, but obviously this is less than ideal. The performance impact increases since each extra page increases read times. The most obvious example of this situation is
VARBINARY(MAX) datatypes. Data type limitations normally relate directly to the size of a page,
(MAX) datatypes effectively bypass the limit and cause overflow pages. For instance,
NVARCHAR(4000) are the normal limits and based on the size of a single page. Using
(MAX) datatypes that span multiple pages increases reads causing less than stellar performance. Queries like
SELECT * can grab poorly performing datatypes accidently and should be avoided as much as possible.
Simply put, extents are groups of pages. Extents consist of exactly eight contiguous pages, with every page being part of an extent. In other words, pages always come in groups of eight, so data grows in a minimum of 64 KB increments. Unlike the many types of pages, there are only two types of extents.
Mixed Extents: In these extents, pages are allocated to multiple objects, or different types of pages. New tables or indexes are put into mixed extents for storage efficiency. When a small table is made that would consist of less than eight pages, it gets stored in a mixed extent with other similarly small objects. If a database grows large enough to fill an entire extent, it can utilize the next type of extent.
Uniform Extents: These extents have pages that are all allocated to the same object. Larger databases often have extents with identical page types, such as data pages or index pages. More data can be read in a single read operation with uniform extents, so performance can see an improvement.
Originally I had planned to provide some example scripts to discover information about your pages, and storage setup, but in an effort to keep the information in byte-sized chunks, I’ll continue with that next week.