An Excerpt from Don Jones’ Definitive Guide to Backup 2.0 about Exchange Server Backups.

SQL Server stores things on disk in 8KB chunks called pages. It also manipulates those same 8KB chunks in memory, meaning the smallest unit of data SQL Server works with is 8KB.

When data is written to disk, an entire row of data must fit within that 8KB page. It’s possible for multiple rows to share a page, but a row cannot span multiple pages. So, if a Customers table has columns for Name, Address, City, State, and Phone, then all that data combined must be less than 8KB. An exception is made for certain data types—such as binary data like photos, or large gobs of text—where the actual page only contains a pointer to the real data. The real data can then be spread across multiple pages, or even stored in a file. SQL Server gathers all these 8KB pages into a simple file on disk, which usually has either an .MDF or an .NDF filename extension.

When SQL Server is told to do something, it’s by means of a query, written in the Structured Query Language (SQL) syntax. In the case of a “modification” query, SQL Server modifies the pages of data in memory. But it doesn’t write those modifications back out to disk yet, as there might be additional changes coming along for those pages and the system load might not offer a good disk‐writing opportunity right then. What SQL Server does do, however, is make a copy of the modification query in a special log file called the transaction log. This file, which has an .LDF filename extension, keeps a record of every transaction SQL Server has executed.
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