DBA 911 – Follow up Q & A

Thanks again to everyone who attended my web session, DBA 911 – Database Corruption, on January 7th. I had a blast spending time with you all. We must do that again, soon. :-)

In this post, I’m going to tackle some of the follow-up questions that we weren’t able to get to in the session. Some questions will require more explanation than others, so I’m going to handle them in separate posts.

First, let’s handle two of the most frequently asked questions:

Where can I get a script to automate running CHECKDB on my databases?

The best scripts for that, bar none, are Ola Hallengren’s. Check out http://ola.hallengren.com

Where can I get the slides and demo code for your presentation?

The slides and demo code are on my resource page, above. (https://dmmaxwell.wordpress.com/resources/)

Now, let’s get into the feedback questions – I’m going to just copy them out of the feedback form and paste them here. I’m going to make an assumption or two here and there, but if I don’t completely answer your question, please feel free to leave a comment below. Some of the questions answered in the presentation will be left alone, but some answers bear repeating, so I will do that here. Also, there are some questions that I want to go into a LOT more detail on, so I’m going to save a few for future posts, where I can include some demo code and explain more.

Here we go! :-)

Is the space taken in tempdb by the dbcc checks cleared after the command is run or does the dba have to manually clear the data?

Space used within TempDB for CHECKDB is cleared automatically by SQL Server. The DBA shouldn’t usually have to manually clear objects out of TempDB. Now, if the problem is that running CHECKDB causes you to run out of space for TempDB, then you need more disk space for TempDB, period. I also recommend pre-sizing TempDB, if at all possible, to the max size you need.

Can we add log file with suspect database if it is suspected due to log file full.

A full log file, meaning a log file that cannot grow, does not put a database into SUSPECT mode. SUSPECT means that SQL Server is not able to read one or more database files to begin with at all, so you wouldn’t be able to add a log file anyway. A suspect database must either be put into EMERGENCY mode in order to correct anything, or restored from backup.

A full log file is a completely different problem. If your log file fills up on an online database, then yes you can add another log file for some temporary space. I would recommend finding out what caused the log file to grow, and either fixing that process or sizing the log file appropriately, then deleting the additional log file.  There’s no good reason to maintain a database with more than one log file.

monitoring the severity alerts > 20, we are monitoring these alerts: 823,824,825 and 832?

For 823, 824 and 832, yes, since those are high-severity alerts. However, 825 is logged quietly as a Severity 10 “Informational” message, so you won’t catch it if you are only monitoring Severity 21 and up. One other reason to monitor specific error numbers outside of just monitoring the severity levels is that you can use custom notification text for each error. For the 823 alert on my servers, I include something like, “This indicates that SQL Server was unable to read an 8k page off of the disk, and that the database is damaged. Please contact the DBA on call to complete a consistency check of this database immediately.”  That way, if I’m not around, someone will see that and know it’s critical – and get a hold of the on call DBA.

how much extra overhead is there with having checksum on for a database

Little to almost none. There is a slight CPU overhead to calculating the checksum. Performance wise, however, most servers are IO-constrained, rather than CPU-constrained. For probably 99.9999% of systems out there, enabling page checksums just isn’t going to be noticed.

I may have missed this but when should you run the datapurity command?

The DATA_PURITY option should be specified when the data purity flag is off for that database.  An example would be a database that is created in SQL Server 2000, then upgraded to 2005 or a later version. In SQL Server 2000, there were no data purity checks. In SQL 2005 and up, the data purity flag was introduced.  To find the data purity flag state, you can use the DBCC DBINFO command, which is included in my demo scripts. Once you have specified DATA_PURITY for a database, that flag will be turned on, so you will get data purity checks automatically going forward, without having to specify that option.

Now, if you’re trying to save time or TempDB space by turning DATA_PURITY off, then you want to specify PHYSICAL_ONLY. Be aware, when you do that, you’re deliberately choosing not to check for logical corruption, but physical readability only. It’s the old speed vs. accuracy trade-off.  I’m not saying one or the other is better, just that you need to understand your options and have reasons for choosing them.

What time will drinks be served?

May I see some ID, please?

What is the licensing implication when you run DBCC on non production environment?

None – you run it wherever you want to. You can copy a production database to a Developer edition server and run CHECKDB on it with no licensing impact.

in What situations is used DBCC UPDATEUSAGE ?

UPDATEUSAGE is a command that verifies allocated space in the database is reported correctly. This has to do with the database catalog views, rather than with corruption or with CHECKDB, in specific. If you’re seeing odd values from sp_spaceused or if  database objects are frequently changed with CREATE, ALTER or DROP statements, then you may want to run this. This does not, however, indicate a corruption issue. I recommend reading the BOL entry for UPDATEUSAGE at http://technet.microsoft.com/en-us/library/ms188414.aspx

Do we need to run CHECKDB after a restore or repair?

If you have the time to run it after a restore, then yes, I would definitely do so. And you should definitely run CHECKDB after doing a repair, especially if you have run with REPAIR_ALLOW_DATA_LOSS. You never want to say to your boss, “Well, I think the database is OK, now.”  You want to be sure. If you have downloaded my demo scripts, you’ll see that I take additional backups both before and after every repair, and run CHECKDB again after every repair. I show this because I think it’s a good habit to have. Your situation may vary, but I would consider this a best practice.  Not only that, but sometimes you need to run multiple repairs, and making it a habit to run CHECKDB after doing one, will allow you to catch anything else that has gone wrong. For an example, see my post on Emergency Mode Repair.

That’s all for now. :-) If I have not answered your question, remember that I’m going to be handling a few other questions separately, in separate posts.  Once I’m done, if I still haven’t answered your question or you have additional questions, feel free to leave a comment, email me, or send me a question on Twitter.   I’m always happy to help.

Thanks for reading.

-David.

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