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It terminates because these critical system catalogs are necessary for DBCC CHECKDB to check the rest of the database. Note that there is no recommended repair level in the DBCC CHECKDB output. These errors cannot be repaired the only option is to restore from your backups. If no problems are found, the next stage is to run database-level allocation consistency checks, which is discussed next.
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Allocation Consistency Checks
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These checks verify the contents of and relationships between the various structures that track page and extent allocations in the database. The structures involved are as follows:
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PFS pages, which track the allocation status of individual pages within a 64-MB section of a data le (a PFS interval) GAM pages, which track the allocation status of all extents within a 4-GB section of a data le (a GAM interval) SGAM pages, which track all mixed extents that have at least one page available for allocation with a GAM interval IAM pages, which track all pages and extents allocated to an allocation unit from a GAM interval IAM chains, which are a linked-list of all IAM pages for an allocation unit (and hence track all pages and extents allocated to the allocation unit from all sections of all data les) The storage engine metadata in the three critical system tables, as described previously
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The allocation consistency checks are very fast in fact, magnitudes faster than the per-table and cross-table consistency checks. The reason for this is that the number of database pages that must be read to perform the allocation consistency checks is magnitudes smaller than the number that must be read to perform all the per-table and cross-table consistency checks.
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Collecting Allocation Facts
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Before any allocation consistency checks can be run, all the necessary information needs to be collected from the various allocation structures and stored as facts. For each data le in each online legroup in the database, the following actions are performed:
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The boot page [page (1:9) in le 1] of the database and the le header page (page 0) in each data le are audited. Error 5250 is reported if this check fails and DBCC CHECKDB terminates. All PFS pages are read and processed. This provides a bitmap of all the IAM pages in the le, as the PFS page also tracks which pages are IAM pages. This also provides
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Microsoft SQL Server 2008 Internals
a bitmap of which pages in the le are from mixed extents. Each set of bitmaps collectively takes less space than all the PFS pages in the le because a PFS page stores 8 bits of information for each data le page.
All GAM pages are read and processed. This provides a bitmap of all the allocated extents in the le. All Shared Global Allocation Map (SGAM) pages are read and processed. This provides a bitmap of all mixed extents in the le that have at least one available page. The Differential Changed Map (DCM) pages and Minimally Logged Map (ML Map) pages are read while the GAM extent is being processed, just to make sure that they can be read correctly. At the start of every GAM interval is a special extent called the GAM extent, which contains the GAM and SGAM pages for that GAM interval. It also contains two other pages that track extents in that GAM interval the DCM and the ML Map pages. The DCM pages track which extents were changed in the GAM interval since the last full database backup. The ML Map pages track which extents were changed in the GAM interval by minimally logged operations since the last transaction log backup.
All IAM pages are read and processed. This provides
A list of all the mixed pages in the le (pages allocated from mixed extents), and by derivation, a list of all mixed extents in the le (remember that the rst IAM page in an IAM chain contains an array the single-page slot array to hold up to eight mixed pages for the allocation unit it represents). A list of all the valid IAM pages in the le. This is necessary because a PFS page may be corrupt and mistakenly have a page marked as an IAM page, or a real IAM page may just be corrupt and unreadable. A list of all the allocated dedicated extents in the le. Linkage information for all IAM chains.
All IAM pages in an IAM chain are linked in a doubly linked list. They also contain a sequence number, starting at 0 for the rst IAM page in the chain and increasing by 1 for each IAM page added to the chain. If any allocation page cannot be read because its header is corrupt, error 8946 is reported (or error 7965 for a corrupt IAM page). This means that a large range of the database is excluded from the consistency checks. The excluded range is reported in error 8998. After all the per- le information gathering, the storage engine metadata is processed as follows: 1. The page ID of the rst IAM page of each IAM chain is stored in the system catalogs (if it wasn t stored somewhere, the storage engine would not know where to nd the list of pages and extents allocated to a table or index).
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