PFS Page Repair

Paul Randal explains why DBCC CHECKDB cannot repair Page Free Space pages:

PFS pages occur every 8088 pages in every data file and store a byte of information about itself and the following 8087 pages. The most important piece of information it stores is whether a page is allocated (in use) or not. You can read more about PFS pages and the other per-database allocation bitmaps in this blog post.

So why can’t they be repaired by DBCC CHECKDB, when all the other per-database allocation bitmaps can?

The answer is that the is-this-page-allocated-or-not information is not duplicated anywhere else in the database, and it’s impossible to reconstruct it in all cases.

In case you’re not particularly familiar with PFS pages, Paul has a blog post from 2006 describing GAM, SGAM, and PFS pages.

Related Posts

Diving Into Spark’s Cost-Based Optimizer

Ron Hu, et al, explain how Spark’s cost-based optimizer works: At its core, Spark’s Catalyst optimizer is a general library for representing query plans as trees and sequentially applying a number of optimization rules to manipulate them. A majority of these optimization rules are based on heuristics, i.e., they only account for a query’s structure and ignore […]

Read More

CHECKDB On Azure SQL Database

Arun Sirpal ponders running DBCC CHECKDB on Azure SQL Database: I was exchanging messages with Azure Support and even though I didn’t get a concrete answer to confirm this I ended up asking the question within a Microsoft based yammer group and yes they do automatically carry out consistency checks. This is great but it […]

Read More

Categories

March 2017
MTWTFSS
« Feb Apr »
 12345
6789101112
13141516171819
20212223242526
2728293031