Tracking DBCC MEMORYSTATUS Over Time

Andy Galbraith has a script to track the outputs of DBCC MEMORYSTATUS over a time period:

Running this statement interactively doesn’t return any data – it just loads the data into DBADatabase.dbo.DBCCMemoryStatus.  Running the commented-out SELECT at the bottom of the script as written will query that table for all rows of counter VM Reserved (virtual memory reserved) but there is much more data than that available if you modify the SELECT.

This query can be dropped into a SQL Agent job step as is and it will run – just like the interactive run it will create the database and permanent table if they don’t exist and then store those nuggets of data into the permanent table for later use – you never know when you may need them!

Click through for the script.

Finding The Last Known Good CHECKDB Run

Amy Herold shows how to find the last known CHECKDB run for each database on a SQL Server instance:

Wednesday I walk into the office and immediately hear that CHECKDB is the source of issues on one of the servers and is the reason behind some errors that have been happening. While I don’t think this is the case (it might look like it on the surface but there is something else that is happening that is the actual cause) I also wanted to find out what CHECKDB was running at the time the errors occurred.

I needed information on when CHECKDB ran for each database. When you look for what you can run to find when CHECKDB was last run you find this blog post and also this blog post on grabbing this info. While these were very informative, they were for one database at a time. I need this for all the databases so I can try to not only find out when each one ran, but also use these time stamps to figure out the duration.

The big recommendation I’d make with regard to this is not to use sp_msforeachdb.  Otherwise, click through for a good script.

Parallel CHECKDB

Kevin Feasel

2017-12-19

DBCC

Arun Sirpal shows us when DBCC CHECKDB can go parallel:

DBCC CHECKDB has the ability to perform parallel checking of objects. However, it absolutely depends on the edition of SQL Server, it only happens when using enterprise edition.

Let’s see this in action. I propose the following tests for this blog post:

  • Test on a SQL Server Enterprise Edition.
  • Test on a non-enterprise edition of SQL Server.

I don’t have 2017 Enterprise at hand but I do have 2014 Enterprise and Express handy so it makes sense to use these versions for my “experiment”.

Read on to see the difference.

Running DBCC CHECKTABLE

Kevin Feasel

2017-11-22

DBCC

Arun Sirpal covers DBCC CHECKTABLE:

If you know about DBCC CHECKDB then most likely you will know about DBCC CHECKTABLE. Quite simply this command performs primitive system-catalog consistency checks, per-table consistency checks on the single table specified, and cross-table consistency checks on indexed views that refer­ence the specified table. (Page 899 Microsoft SQL Server Internals 2012, Chapter 14, Page 899, Paul Randal)

There is one concept about this command that I did not know about until recently, intrigued? Read on.

I was going to say “read on” but Arun already used that line.

CHECKDB On Azure SQL Database

Kevin Feasel

2017-09-05

Cloud, DBCC

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 is one less thing for me to worry about and if there is serious corruption, you know potential data loss (which would be rare) then they will definitely tell you and work with you.

However, it doesn’t mean you CAN’T run it, I was curious so I ran DBCC CHECKDB on my Azure SQL Databases, but like with any other consistency check it is best to do it OFF-PEAK hours. I would probably take it a step further and wouldn’t even bother running it.

It’s an interesting post, reminding us that administering an Azure database isn’t the same as on-prem.

When CHECKDB Snapshots Run Out Of Disk Space

Andy Galbraith walks through an error message in DBCC CHECKDB when the snapshot runs out of disk space:

Looking in the SQL Error Log there were hundreds of these combinations in the minutes immediately preceding the job failure:

The operating system returned error 665(The requested operation could not be completed due to a file system limitation) to SQL Server during a write at offset 0x000048a123e000 in file ‘E:\SQL_Data\VLDB01.mdf:MSSQL_DBCC17‘. Additional messages in the SQL Server error log and system event log may provide more detail. This is a severe system-level error condition that threatens database integrity and must be corrected immediately. Complete a full database consistency check (DBCC CHECKDB). This error can be caused by many factors; for more information, see SQL Server Books Online.

Error: 17053, Severity: 16, State: 1.

E:\SQL_Data\VLDB01.mdf:MSSQL_DBCC17: Operating system error 665(The requested operation could not be completed due to a file system limitation) encountered.

Read on for more information, including a rough idea of how much space the snapshot requires as well as a few workarounds and hints.

When CHECKDB Doesn’t Catch Corruption

Kevin Feasel

2017-07-20

DBCC

Paul Randal explains one condition when DBCC CHECKDB misses corruption:

An interesting situation was discussed online recently which prompted me to write this post. A fellow MVP was seeing periodic corruption messages in his error log, but DBCC CHECKDB on all databases didn’t find any corruptions. A subsequent restart of the instance caused the problem to go away.

My diagnosis? Memory corruption. Something had corrupted a page in memory – maybe it was bad memory chips or a memory scribbler (something that writes into SQL Server’s buffer pool, like a poorly-written extended stored procedure), or maybe a SQL Server bug. Whatever it was, restarting the instance wiped the buffer pool clean, removing the corrupt page.

So why didn’t DBCC CHECKDB encounter the corrupt page?

Read on for the rest of the story.

Finding Last DBCC Command Runs

Andrew Kelly has a script to find the last time somebody ran a DBCC command like DBCC FREEPROCCACHE:

Let me explain a few things about the script. I am getting the path of the current trace file and placing it into a variable. The current file name will almost certainly have a suffix of _nn just before the .trc extension.  If I were to run the script as is I would only be reading the current log file and not the other 4 that preceded it. If all you care about is the current log file then fine but most will want to search all the existing log files. One way to do this is to simply replace the current file name with just log.trc and use default as the 2nd parameter as I did above in the fn_trace_gettable function. The default parameter value tells the function to read all files from that one onward. even though log.trc doesn’t actually exist it knows how to handle it and reads all of the existing trace files in order.

So if the string that we search on (here we use ‘dbcc free%’) is in any of the files it will return the matching rows. You may have to adjust the wildcards and such but I think you get the idea. Again remember that the data is transient so always look at the StartTime column in the logs to ensure you know which Date and Time range you are looking at. You can do something like this but I will leave that up to you.

SELECT MIN(StartTime) AS [Begin], MAX(StartTime) AS [End]  FROM ::fn_trace_gettable(@Path,default)

A word of caution in that I never bothered to see just how resource intensive this function is. while I don’t expect any issues with normal use it is not something you want to be searching on every second. Be sensible and you should have no problems.

Click through for more details, including the script Andy uses to do this search.

Duplicate Key Error On DBCC CLONEDATABASE

Kevin Feasel

2017-06-15

Bugs, DBCC

Erin Stellato shows an edge case when you have a rather old database you’re trying to clone:

If you’ve been using DBCC CLONEDATABASE at all, you might have run into a cannot insert duplicate key error (or something similar) when trying to clone a database:

Database cloning for ‘YourDatabase’ has started with target as ‘COPY_YourDatabase’.
Msg 2601, Level 14, State 1, Line 1
Cannot insert duplicate key row in object ‘sys.sysschobjs’ with unique index ‘clst’. The duplicate key value is (1977058079).

If you do some searching, you’ll probably end up at this Connect item: DBCC DATABASECLONE fails on sys.sysowners.

The Connect item states that the problem exists because of user objects in model.  That’s not the case here.

I’m working with a database created in SQL Server 2000…now running on SQL Server 2016.

This isn’t very likely to pop up for most places (I hope!), but it’s good to know.

Incorrect PFS Free Space

Dave Mason walks through troubleshooting one database corruption scenario:

I’ve been lucky with database corruption during my career. I could probably count on one hand the number of times I’ve had to deal with it. A couple times, it was in a customer’s environment–they managed it themselves, but called me in to help. The other incidents were ones I inherited from a backup I had to restore into a production environment. The first time it happened to me, I didn’t realize it until days later when DBCC CHECKDB ran during a weekend maintenance window. After that, I added a new “rule” to my list: always run DBCC CHECKDB after restoring a database from someone else. That rule paid dividends today.

Here’s the output I saw:

Msg 8914, Level 16, State 1, Line 50
Incorrect PFS free space information for page (1:2564368) in object ID 457768688, index ID 1, partition ID 72057619124060160, alloc unit ID 72057594116767744 (type LOB data). Expected value 0_PCT_FULL, actual value 100_PCT_FULL.
CHECKDB found 0 allocation errors and 1 consistency errors in table 'tbl_Redacted' (object ID 457768688).
CHECKDB found 0 allocation errors and 1 consistency errors in database 'db_redacted'.
repair_allow_data_loss is the minimum repair level for the errors found by DBCC CHECKDB (db_redacted).

Read on to see how Dave solved this issue.

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