SQLServerBang

SQL Server IAM page Hi Guys, Welcome to SQL Server allocation series, hope you enjoy the  post1 and post2 . Today, I’m going to show you the detail of IAM page. An  Index Allocation Map (IAM)  page maps the extents in a 4-GB part of a database file used by an allocation unit. An allocation unit is one of three types: IN_ROW_DATA Holds a partition of a heap or index. LOB_DATA Holds large object (LOB) data types, such as XML, VARBINARY(max), and VARCHAR(max). ROW_OVERFLOW_DATA Holds variable length data stored in VARCHAR, NVARCHAR, VARBINARY, or SQL_VARIANT columns that exceed the 8,060 byte row size limit. Each partition of a heap or index contains at least an IN_ROW_DATA allocation unit. It may also contain a LOB_DATA or ROW_OVERFLOW_DATA allocation unit, depending on the heap or index schema. An IAM page covers a 4-GB range in a file and is the same coverage as a GAM or SGAM page. If the allocation unit contains extents from more than one file, or more than one 4-GB range of a file, t

In previous post , we discussed the PFS, GAM and SGAM pages. Today, I’m going show you two related corruptions on these pages.   SQL Server checks the bit in PFS, GAM, SGAM and IAM are checked when new pages are allocated. It guarantees that it does not store data to a wrong page/extent. These pages are crucial to SQL Server, SQL Server considers it as a corruption if they are messed up. I’m going to use the same database I used in previous article . You may restore the   backup  f ile and give it a try. Please note, restoring database will consume some pages and change the PFS, GAM, SGAM and IAM. You may get a different view from my mine.   Let me show you some of the content of GAM and SGAM before the corruption samples. Here are the GAM and SGAM page result: 1.GAM 2.SGAM 3.We can tell that the extent of page(1:352) and extents greater than that have not been allocated. 4.Let’s take a dive into the GAM page. 5.‘ 00000000 00f0 ’ interpretation: 5)Pa