Feeds:
Posts
Comments

Archive for the ‘DBA Interview’ Category

Interesting one today:

On one of our production machines, we recently added a new LUN to a SQL cluster. A task like this is a team effort. Sysadmins perform some steps and DBA carry out the remaining. In this article, the main focus is on covering the steps after the LUN is added to the OS & Sql Cluster by the sysadmins.  For context, we’ll start with high level steps before going into details.

Sysadmins steps

  1. Add new storage to the machine/OS as an available storage
  2. Format the available drive with appropriate settings (cluster size) and add it as a new drive
  3. Make drive available to the Cluster using “Add Disk” screen in FailOver Cluster Management tool.

DBAs steps

  1. Add available storage to Sql Cluster
  2. Configure dependency (and check the report before & after)
  3. Add data file on the new cluster storage drive

Here we’ll cover the DBA steps in detail:

Some of these steps were covered under a different recent article as part of dealing with an error message, but here we’ll cover it as a task by itself (which it is).

Add New Storage

Once sysadmins have made the new storage as an ‘available storage’ to OS Cluster, it needs to be added as a new storage location to the SQL Cluster.

In FailOver cluster manager, go to Sql Server Resource Group for this SQL Cluster and right click for detailed options and choose “Add Storage” (see image below)

sqlcluster_addnewstorage_to_os_cluster

Once successful, go to Storage\Disks under in FailOver Cluster Manager to confirm the availability. See image below:

sqlcluster_addnewdrive

Configure Dependency

Adding the storage is an important step, and equally important step is adding the new drive to Sql Cluster Dependency Chain. Dependency Chain informs Sql Sever “how to act”, when any resource in the Cluster becomes unavailable. Some resources automatically trigger cluster failover to other node; some resources do not. This decision is made based on the configurations in Dependency Chain.

Example:

Critical: Data drive/LUN that has database files is critical for optimal availability of the Sql Cluster. So, if it becomes unavailable, failing over to other available nodes is imperative to keep the cluster available.

Non-Critical: In some scenarios, Sql Server Agent is not considered as Critical. So if it stops for some reason, Cluster will make multiple attempts to start it on the same node, but may not necessarily cause failover.

This is a business decision. All these “response actions” will be configured in Cluster settings.

Now, check the dependency report (before); We can see that new drive exists in Cluster, but is not yet added to the Dependency Chain.

SqlCluster_DependencyReport.PNG

To Configure Dependency Chain, go to the Sql Server Resource Group under Roles in FailOver Cluster Manager. See the image below for clarity:

Then go to the bottom section for this Resource Group, where all the individual resources that are part of this Resource Group are displayed.

Under “Other Resources“, right click on Sql Server Resource and choose properties.

do As show

sqlcluster_addnewstorage_add_to_dependency

In the “Sql Server Properties” window, we can see the existing resources already added to dependency chain logic.

SqlCluster_Dependency_Before.PNG

Now, go to the end of the logic list and choose “AND” for condition and pick the new Cluster Storage to be included. See image below for clarity:

SqlCluster_Dependency_After.PNG

After saving the settings, regenerate the Dependency Chain report. Now, we’ll see the new drive as part of the logic.

sqlcluster_dependencyreport_after

Add Database Data File to New Cluster Storage

Now, that the new drive is ready, we could easily add a new data file to the new location.

--
-- Add data file to new storage location
--
USE [master]
GO
ALTER DATABASE [SampleDB]
ADD FILE
	(
		  NAME 			= 	N'SampleDB_Data3'
		, FILENAME 		= 	N'U:\MSSQL\Data\SampleDB_Data3.NDF'
		, SIZE 			= 	3500 GB
		, FILEGROWTH 	= 	100 GB
		, MAXSIZE 		= 	3900 GB
	)
TO FILEGROUP [PRIMARY]
GO
Hope this helps,
_Sqltimes

Read Full Post »

Quick one today:

Earlier today, we ran into this error in our lab environment with replication. Two different errors, on the same environment, due to similar sequence of events.

Msg 20584, Level 16, State 1, Procedure sp_MSrepl_check_server, Line 67
Cannot drop server because it is used as a Subscriber to remote Publisher in replication.

Cannot drop server because it is used as a distributor in replication

This type of errors are rare, but one of these three options below might provide some relief.

  1. Reset Dist flag
  2. Force drop distributor
  3. Manually remove Publishers (from Distributor properties)

Also, see the notes in the bottom section (points to keep in mind)

Reset Dist Flag for Distributor Instance

Sometimes, the dist flag for Distributor stays set, when not supposed to. So manually resetting it helps.

--
--  Reset Distributor flag
--
EXEC master.dbo.sp_ServerOption   @server = 'REPLDist'
				, @OptName = 'dist'
				, @OptValue = 'false'
GO

Force Drop Distributor

One of the last options is to ‘force’ drop distributor. This allows removing distributor database without running through the proper checks. This step is a high risk, high reward option; Since all checks were previously performed and all components of replication are removed, except distributor, this step might provide relief.

EXEC sp_dropdistributor @no_checks = 1, @ignore_distributor = 1
GO

Manually Remove Publishers

Usually, the above two steps will resolve the issue. But in rare occasions, the issue persists, like in our lab. For those stubborn situation, this technique has given some relief.

Go to Replication >> Distributor Properties >> Go to Publishers (tab)

Now, un-check each Publisher and save. Important to note that, this needs to be done for one publisher at a time.

Distributor_Properties_RemovePublishers.PNG

Points to keep in mind:

Sometimes, just the one of these steps might be enough to resolve the issue; Sometimes, it might take two of the steps. In our lab, we ended up needing all the 3 steps. the first step is easy to implement.
The second and third steps might need repeated attempts. Something like, try Step 2, if it fails, then try step 3; If Step 3 succeeds, then go back to Step 2. Once Step 2 is successful, the distributor is fully cleared and cleaned.
Hope this helps,
_Sqltimes

Read Full Post »

Quick one today:

Years ago, there was a popular comment that Sql Server did not wide variety of functions; Once of them is the much needed IsNumeric functionality. Though such a function existed, there was a case where it resulted in incorrect results. So there were several custom functions like IsReallyNumeric or IsInteger, etc (my personal DBA library also had such custom functions)

Now, there is a powerful function that does more than just IsNumeric capability. It is called TRY_PARSE. Let’s take an example:

--
-- Sample code for TRY_PARSE
--
SELECT	  TRY_PARSE('ABCDEF' AS INT)		AS [INT_Test1]
	, TRY_PARSE('123456' AS INT)		AS [INT_Test2]
	, TRY_PARSE('ABCD-01-01' AS DATETIME)	AS [DateTime_Test1]
	, TRY_PARSE('2016-01-01' AS DATETIME)	AS [DateTime_Test2]
GO

TRY_PARSE.PNG

This function takes input in VARCHAR/NVARCHAR format and attempts to convert into the requested format. If the result is successful, the value is returned in requested data type. If not, NULL is returned. This makes it very useful in data processing logic flows; And makes it more useful function than the legacy (IsNumeric) functions.

Hope this helps,
_Sqltimes

Read Full Post »

Interesting one today:

MSDTC is one of the popular software components that is on all Windows systems. It is one of the Windows  Operating System components that Sql Server relies on it to perform some crucial tasks (when needed).

What does it do?

MSDTC, Microsoft Distributed Transaction Coordinator, is essentially, as name suggests, a coordinator/manager to handle transactions that are distributed over multiple machines. Let’s say we start a transaction, where one of the steps includes querying data from a different Sql Server instance on a different physical machine; MSDTC comes into action with these specific tasks that need transaction coordination across different physical machines. It executes the section of code that is supposed to run on remote machines and brings back the results to local Sql instance. In this process, if any issue were to occur, on the remote machine that results in rollback, MSDTC makes sure the original transaction on this machine also rolls-back safely.

How does it do?

MSDTC comes with necessary Operating System controls and memory structures to carry out these operations independent of the Sql Instances, while keeping integrity of the transaction across the multiple physical Sql machines a.k.a. the complete two-phase distributed commit protocol and the recovery of distributed transactions.

Where does Sql Server use it?

The key point here is that these need to be Sql Instances on different physical machines. Queries that request data across different instances on the same physical box do not go through MSDTC.

MSDTC is used by query activities like

  • Linked Servers
  • OPENROWSET
  • OPENQUERY
  • OPENDATASOURCE
  • RPC (Remote Procedure Calls)
  • Ones with
    • BEGIN DISTRIBUTED TRANSACTION
  • etc…

So, every time we run SQL queries that utilize above techniques, they rely on MSDTC to carry out operation while maintaining transaction integrity.

Who else uses it?

MSDTC is an Operating System resource that is used by applications other than Sql Server, to perform any distributed transaction activities; Like eXtended Architecture applications.

Is MSDTC required?

MSDTC is not required for Sql Server installation or operation. If you are only going to use Database Engine, then it is not required or used. If your Sql uses any of the above mentioned query techniques (Linked Server, OPENQUERY, etc), or SSIS or Workstation Components then MSDTC is required.

If you are installing only the Database Engine, the MSDTC cluster resource is not required. If you are installing the Database Engine and SSIS, Workstation Components, or if you will use distributed transactions, you must install MSDTC. Note that MSDTC is not required for Analysis Services-only instances.

What about Sql Cluster?

Same rules as above apply to Sql Clusters as well with one additional rule. If you have two instances on the same machine (that are clustered across different physical machines), then you’ll need MSDTC. Since the Cluster could failover to remote machine at anytime.

Let’s take an example:

Let’s say Instance1 is on physical machines A & B, with B as active node. Instance2 is on machines B & C, with B as active node. A query going from Instance1 to Instance2 will need MSDTC (even if both the instances are active on the same physical machine B at that given point in time.).

This is because, there is no guarantee that they will remain on the same physical machine at any given time; They might failover to other machines, resulting in instances being on physically different machines. So MSDTC is required (when distributed operations are performed).

Also the recent Sql Server versions do not required MSDTC during Sql Server installations.

Other points in a Clustered Environment

We could have multiple instances of MSDTC as different clustered resource (along with default MSDTC resource).

In scenario with multiple MSDTC, we could configure each Sql Cluster resource to have a dedicated MSDTC instance. If such mapping does not exist, it automatically falls back to use the default MSDTC resource.

Hope this helps,
_Sqltimes

Read Full Post »

Interesting one today:

Context:

In replication, as we all know, data/records from Publisher are copied to all Subscribers. This is carried out as Transactions and Commands from Publisher to each Subscriber. Let’s say application INSERTs 10 new rows into the database (Publisher) as part of some application transaction; Now the LogReader reads Transactional Log file and takes a note of these 10 records; Brings their details to Distributor.

Behind the Scenes:

From Distributor, they are transferred to each Subscriber by the Distribution Agent. In actuality, the Distribution Agent executes these INSERT commands at each subscriber; Upon success, it proceeds to execute the subsequent commands until all Subscribers are up to date.

Here is the beauty:

In Replication, the result of INSERT activity by the application, is carried out as “1 transaction with 10 commands” (one INSERT command for each record, all encompassing in one transaction). When we look closely, each INSERT command is carried out by Replication Stored Procedures located in each Subscriber (they are created as part of replication setup). Distributor will have entries with these stored procedures with appropriate parameters. Now Distribution agent will execute them at each subscriber.

Distributor will have entries like:

--
--
--
{CALL [dbo].[mtx_rplins_SampleTable] (758374661,1,N'SomeName1')}
{CALL [dbo].[mtx_rplins_SampleTable] (758374662,4,N'SomeName2')}
{CALL [dbo].[mtx_rplins_SampleTable] (758374663,5,N'SomeName3')}

 

replication_storedprocedures

Question

Now here is the question that we’ll discuss today. Why do we have different replication stored procedures calls? Not different procedures, but different procedure calls. We have dedicated replication stored procedures for each table; Each to carry out INSERT, UPDATE and DELETE. Like

  • sp_MSins_ : Handles INSERT operations for this table
  • sp_MSupd_ : Handles UPDATE operations for this table
  • sp_MSdel_ : Handles DELETE  operations for this table

But why do we have different procedure calls?

In the above image, we see entries like {CALL […..]}. There other procedure calls like CALL, SCALL, XCALL, MCALL, VCALL. What are these for?

  • CALL
    • This is the most popular or default call used in Replication.
    • It could be used for INSERT or UPDATE or DELETE
  • SCALL
    • This is used for UPDATEs only.
  • XCALL
    • Used for UPDATEs and DELETEs
  • MCALL
    • Used for UPDATEs only
  • VCALL
    • For internal use only.
    • Also used for updatable subscriptions

CALL:

This is the default option used for all activities a.k.a. UPDATE/DELETE/INSERT. As seen in the picture above, CALL will have a list of values provided for all columns of the table.

SCALL & MCALL:

Looks like some procedures are used exclusively for certain actions; Like MCALL & SCALL are dedicated for UPDATEs. But there is some difference between these too. SCALL only sends the columns that need updating; Where as MCALL sends all columns of the table (including the ones that are not changing).

XCALL:

Sends data for both before and after values for all columns of the table involved in UPDATE or DELETE. Obviously, in UPDATE, there are values for both before and after; But in DELETE, the after values is empty.

For more details, please refer to this excellent article.

Hope this helps,
_Sqltimes

Read Full Post »

Interesting article today on troubleshooting replication errors.

A few weeks ago, on production, we received an alert with on replication failures. Upon further inspection the error looks like this:

Command attempted:
if @@trancount > 0 rollback tran
(Transaction sequence number: 0x001031C200001B06000700000000, Command ID: 1)

Error messages:
The row was not found at the Subscriber when applying the replicated command. (Source: MSSQLServer, Error number: 20598)
Get help: http://help/20598
The row was not found at the Subscriber when applying the replicated command. (Source: MSSQLServer, Error number: 20598)
Get help: http://help/20598

Identify Root Cause:

To understand the course of action, we need to first understand the underlying issues. Go to Replication Monitor >> Open Details Window on the subscription that has errors. Go to ‘Distributor to Subscriber’ tab for more details. See the image below:

replication_error_20598

Now we see that, replication is attempting to perform an action on the Subscriber, but the row does not exist. So, lets find out more.

Find the exact command that is being replicated (or executed on Subscriber as part of replication) that throws this error. Use replication procedure sp_browsereplcmds for that.

Query the Distribution agent ID from dbo.MSdistribution_agents and use it in the query below.

--
-- Uncover the replication command throwing error
--
EXEC sp_browsereplcmds @xact_seqno_start = '0x001031C200001A620004000000000000'
                     , @xact_seqno_end = '0x001031C200001A620004000000000000'
                     , @agent_id = 49
                     , @publisher_database_id = 3
GO

You’ll see something like this:

replication_error_20598_investigation

Under the command column, we’ll see the exact command that is running into this error.

--
--  Error 20598 occurring in
--
{CALL [mtx_rpldel_ReportCriterion] (908236,71357,250,-1)}

Now, lets go to that stored procedure ‘mtx_rpldel_ReportCriterion’ and see what tables are involved in manipulation. In my scenario, the table ReportCriterion does not have the record with ID = 908236

Resolution

Once you understand the root cause, we have a few options.

  1. Data Integrity: Looks like we have synchronization issues between Publisher and Subscriber. If it is a non-production environment or an environment where reinitializing is an option, then we could take that route to sync up the data first.
    1. Once data integrity issues are resolved, all subsequent replication commends would be successful.
  2. Manual fix: Manually insert the missing record at Subscriber and then allow replication to perform its operations on the new record.
    1. With this option, the more records we uncover as missing, the more manual operation would be required. Its not ideal, but it is a workaround to get things going again.
  3. Ignore, for now: In some situations, until further solution is identified, we may need to ignore this one record and move forward with rest of the replication commands.
    1. Take necessary precautions to make sure there are no more such missing records. Or gather a list of all missing ones.
    2. Configure replication to ignore error 20598 using skiperrors parameter. There are a couple of ways to achieve this; here we’ll look at one.
    3. Go to the Agent Profile for this particular Distributor Agent. One of the profiles allows us to skip certain errors. See the image below.
    4. replication_error_20598_resolution

For more information, please refer to Microsoft Support article on similar issue.

Hope this helps,
_Sqltimes

Read Full Post »

Older Posts »