Blocking A Truncate Statement

Arun Sirpal shows that the TRUNCATE command needs to take locks like any other data modification command:

The truncate option is fast and efficient but did you know that it takes a certain lock where you could actually be blocked?

What am I talking about? When you issue a truncate it takes a Sch-M lock and it uses this when it is moving the allocation units to the deferred drop queue. So if it takes this lock and you look at the locking compatibility matrix below you will see what can cause a conflict (C).

Arun includes an image which shows what can block what, and also shows us an example.

The Blocking Monitoring Framework

Dmitri Korotkevitch announces a new tool:

Troubleshooting of the blocking and concurrency issues is, in the nutshells, a simple process. You need to identify the processes involved in blocking conditions or deadlocks and analyze why those processes acquire the locks on the same resources. In majority of cases, you need to analyze queries and their execution plans identifying possible inefficiencies that led to excessive number of locks being acquired.

Collecting this information is not a trivial task. The information is exposed through DMVs (you can download the set of scripts here); however, it requires you to run the queries at time when blocking occurred. Fortunately, SQL Server allows you to capture blocking and deadlock conditions with the blocked process report and deadlock graph, analyzing them later.

There is the caveat though. Neither blocked process report nor deadlock graph provide you execution plans of the statements. Nor do they always include affected statements in the plain text. You may need to query plan cache and other DMVs to get this information and longer you wait lesser is the chance that the information is available. Moreover, SQL Server may generate enormous number of blocked process reports in cases of prolonged blocking and complex blocking chains, which complicates the analysis.

Confirmed to work with SQL Server 2012 and later, but might work on earlier versions as well.  Dmitri has released it to the public, so check it out.

Viewing Deadlock Graphs With The system_health Session

Jes Borland shows how you can avoid using trace flags 1204 and 1222 and view deadlocks from the system_health Extended Event:

This one isn’t bad, but imagine a multi-statement deadlock, or a server with several deadlocks in an hour – how do you easily see if there were other errors on the server at the same time?

With SQL Server 2012+, we have a better tool to see when deadlocks occur – and the deadlock graphs are saved by default, so we don’t have to read the text version to figure it out, or run a separate trace to capture them.

In SSMS, open Object Explorer and navigate to Extended Events > Sessions > system_health > package0.event_file. Double-click to view the data.

Click through for the entire process.

Query Store Cleanup Can Be Blocked

Kendra Little shows that you can block Query Store cleanup:

This is an isolated test system, so I went to clean out Query Store as a reset. I didn’t need any of the old information in there, so I ran:

  • ALTER DATABASE BabbyNames SET QUERY_STORE CLEAR ALL;
  • GO

I was surprised when I didn’t see this complete very quickly, as it normally does.

Click through to see how Kendra diagnoses the issue.

OLTP-Friendly Database Deployments

Michael Swart looks at one of the biggest problems when trying to do a zero-downtime deployment to an OLTP system:

There are two main kinds of SQL queries. SELECT/INSERT/UPDATE/DELETE statements are examples of Data Manipulation Language (DML). CREATE/ALTER/DROP statements are examples of Data Definition Language (DDL).

With schema changes – DDL – we have the added complexity of the SCH-M lock. It’s a kind of lock you don’t see with DML statements. DML statements take and hold schema stability locks (SCH-S) on the tables they need. This can cause interesting blocking chains between the two types where new queries can’t start until the schema change succeeds

Click through for suggestions with regard to schema locks, as well as a few tips for modifying large tables.

What Update Locks Do

Guy Glantser explains the process around updating data in SQL Server, particularly the benefit of having update locks:

In order to update a row, SQL Server first needs to find that row, and only then it can perform the update. So every UPDATE operation is actually split into two phases – first read, and then write. During the read phase, the resource is locked for read, and then it is converted to a lock for write. This is better than just locking for write all the way from the beginning, because during the read phase, other sessions might also need to read the resource, and there is no reason to block them until we start the write phase. We already know that the SHARED lock is used for read operations (phase 1), and that the EXCLUSIVE lock is used for write operations (phase 2). So what is the UPDATE lock used for?

If we used a SHARED lock for the duration of the read phase, then we might run into a deadlock when multiple sessions run the same UPDATE statement concurrently.

Read on for more details.

ROWGROUP_FLUSH Deadlocks Inserting Into Clustered Columnstore Indexes

Joe Obbish ran into a strange deadlock when performing concurrent insertions into a clustered columnstore index:

We’ve only observed this deadlock with multiple concurrent sessions insert to the delta store for the same target CCI due to server memory pressure or very low cardinality estimates (less than 251 rows). The correct mitigation depends on why you’re seeing the issue in the first place. If you’re seeing it due to low cardinality estimates then fix your estimates, or at the very least get them above 250 rows. If you’re seeing them because the memory grant for the CCI build times out after 25 seconds then lower concurrency or increase server memory.

The problem can also be avoided by not doing concurrent inserts in the first place. In some cases a parallel insert may be a reasonable alterative. There’s also some evidence that the deadlock is only seen when the number of rows for insert is very close to 1048576, but we weren’t able to make any definitive conclusions around that.

Read the whole thing.  Also check out his Connect item.

Finding Blocking In SQL Server

Amy Herold has a script to help you find which query is blocking your important query:

It might look complicated but it is actually very simple – query sys.sysprocesses with a cross apply using the sql_handle to get the text of the query, and then an outer apply with the same query again but you are joining to the blocking spid so you can get the text for the query that is doing the blocking. Beyond that, you can filter on various columns and refine your output

Andy Mallon goes one step further and searches for the lead blocker:

When blocking goes bad, it can go really bad. Sometimes it’s because someone (usually, that someone is me) forgets to commit a transaction before going to lunch, and those open locks cause a bunch of blocking. Sometimes a data load runs at a strange time, or an unusual amount of data gets loaded, or a query gets a bad plan and starts running long, or… you get the idea. There are a bunch of reasons this can come up.

The hardest part is that sometimes big blocking chains build up. The session I forgot to commit blocks 5 session. Each of those block 5 sessions. Each of those block 5 sessions… Eventually, I have 8000 sessions waiting on me, and I’m off eating a kale & farro salad. Oops.

The moral of the story is, don’t eat kale and farro salads; that sounds like rabbit food.

NOLOCK Doesn’t Mean No Locks

Bert Wagner points out that SELECT queries with NOLOCK can still cause blocking to occur:

This is where my understanding of NOLOCK was wrong: while NOLOCKwon’t lock row level data, it will take out a schema stability lock.

schema stability (Sch-S) lock prevents the structure of a table from changing while the query is executing. All SELECT statements, including those in the read uncommitted/NOLOCK isolation level, take out a Sch-S lock. This makes sense because we wouldn’t want to start reading data from a table and then have the column structure change half way through the data retrieval.

However, this also means there might be some operations that get blocked by a Sch-S lock. For example, any command requesting a schema modification (Sch-M) lock gets blocked in this scenario.

Read on to see which types of commands take schema modification locks, and ways to minimize the pain.

Columnstore Deadlocking

Kendra Little shows us a scenario in which querying columnstore metadata during table updates can lead to a deadlock:

I was playing around with a nonclustered columnstore index on a disk-based table. Here’s what I was doing:

Session 1: this session repeatedly changed the value for a single row, back and forth. This puts it into the delta store.

Session 2: this session repeatedly counted the number of rows in the table, using the columnstore index.

With my sample data in this scenario, I found I frequently generated deadlocks.

Read on for more.

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