dbcc

DBCC Config

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Dbcc Config is an undocumented DBCC command which returns information of server configuration, which presented in DMV sys.configurations, on file 1, page 10 in master database. This command will return nothing if you don’t have trace flag 3604 on.

dbcc traceon(3604)
dbcc config --with tableresults
dbcc page(master,1,10, 3) -- with tableresults

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Freeze and Unfreeze Write IO on a Database

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Do you know you can suspend the write IO on one of your database and resume them later? 2 undocumented DBCC command allow you to do so, DBCC Freeze_IO and DBCC Thaw_IO. When DBCC Freeze_IO is called all the IO performing writing operation will be suspended immediately (not terminated). Once the IO is frozen, you can treat the database as a read-only database. Any writing activity will not return like you write data to a readonly database, they hang until you unfreeze the io by calling DBCC Thaw_IO.

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DBCC WritePage

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DBCC WritePage is a dangerous undocumented DBCC command. It’s useful in your sandbox to change data, crash objects, and do some fun things you may think of. But never ever use it in your productions. Using DBCC WritePage command is at your own risk. . I will give you demos on how to use dbcc writepage to perform simple insert, delete, and update on a table.

dbcc writepage ({'dbname' | dbid}, fileid, pageid, offset, length, data [, directORbufferpool])

{‘dbname’ | dbid}
Database name or database id
fileid
File id in sys.database_files
pageid
Page number in the file
offset
Starting position of the data you want to change
length
number of bytes to be written to the page
data
data to be written. It’s binary, for example 0x13432
directORbufferpool
2 possible values, 0 and 1. When it’s 0, which is also the default value, the data modification is done at buffer level, related data will be calculated automatically such as page hash. When it’s 1, the modification is written to disk directly, related values, such as page hash, will not be recalculated automatically. Unless you want to test your math level…

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