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Displaying posts with tag: locking (reset)
Differences between READ-COMMITTED and REPEATABLE-READ transaction isolation levels

As an instructor with Percona I’m sometimes asked about the differences between the READ COMMITTED and REPEATABLE READ transaction isolation levels.  There are a few differences between READ-COMMITTED and REPEATABLE-READ, and they are all related to locking.

Extra locking (not gap locking)
It is important to remember that InnoDB actually locks index entries, not rows. During the execution of a statement InnoDB must lock every entry in the index that it traverses to find the rows it is modifying. It must do this to prevent deadlocks and maintain the isolation level.

If you run an UPDATE that is not well indexed you will lock many rows:

update employees set store_id = 0 where store_id = 1;
---TRANSACTION 1EAB04, ACTIVE 7 sec
633 lock struct(s), heap size 96696, 218786 row lock(s), undo log entries 1
MySQL thread id 4, OS thread handle 0x7f8dfc35d700, query id 47 localhost root …
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SQL Locking and Transactions – OSDC 2011 video

This recent session at OSDC 2011 Canberra is based on part of an Open Query training day, and (due to time constraints) without much of the usual interactivity, exercises and further MySQL specific detail. People liked it anyway, which is nice! The info as presented is not MySQL specific, it provides general insight in how databases implement concurrency and what trade-offs they make.

See http://2011.osdc.com.au/SQLL for the talk abstract.

InnoDB locking makes me sad

Vadim and others have pointed at the index->lock problems before, but I think they didn’t good job enough at pointing out how bad it can get (the actual problematic was hidden somewhere as some odd edge case). What ‘index lock’ means is generally the fact that InnoDB has table-level locking which will kill performance on big tables miserably.

InnoDB is a huge pie of layers, that have various locking behaviors, and are layered on top of each other, and are structured nicely as subdirectories in your innodb_plugin directory. Low level storage interfaces are done via os/ routines, then on top of that there’s some file space manager, fsp/, which allocates space for btr/ to live in, where individual page/ entities live, with multiple row/ pieces. There’re few other subsystems around, that got …

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A few notes on locking in MySQL

This is another article in a series of articles titled "A few notes ..." in which I will be posting some important information about locking concepts, different types of locks and what locks table engines support. Just like the previous article, the purpose of this article is to highlight important aspects that you should have in the back of your mind when developing applications.

Understanding InnoDB transaction isolation levels

Isolation is an important part of ACID properties that guarantee that transactions are processed in a reliable manner. But there are four different levels of isolation available and you have to understand each one of them to be able to select the correct one for your needs. This post intends on explaining the four levels together with their effects on locking and performance.

The Casual MySQL DBA – Operational Basics

So your not a MySQL DBA, but you have to perform like one. If you have a production environment that’s running now, what are the first things you do when it’s not running or reported as not running?

  1. Are the MySQL processes running? (i.e. mysqld and mysqld_safe)
  2. Can you connect locally via cli?
  3. What’s in the MySQL error log?
  4. What are current MySQL threads doing? Locked? long running? how many? idle sources?
  5. Can you connect remotely via cli?
  6. Verify free diskspace?
  7. Verify system physical resources?
  8. If this is a slave, is MySQL replication running? Is it up to date?
  9. What is the current MySQL load, e.g. reads/writes/throughput/network/disk etc?
  10. What is the current InnoDB state and load? (based on if your using InnoDB)

After you do this manually more then once you should be scripting these commands to …

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Why do I recommend switching over from MyISAM to Innodb!

Although MyISAM has been the default storage engine for MySQL but its soon going to change with the release of MySQL server 5.5. Not only that, more and more people are shifting over to the Innodb storage engine and the reasons for that is the tremendous benefits, not only in terms of performance, concurrency, ACID-transactions, foreign key constraints, but also because of the way it helps out the DBA with hot-backups support, automatic crash recovery and avoiding data inconsistencies which can prove to be a pain with MyISAM. In this article I try to hammer out the reasons why you should move on to using Innodb instead of MyISAM.

Common MySQL Scalability Mistakes

This week I was one of the presenters at the first Surge Scalability Conference in Baltimore. An event that focused not just on one technology but on what essential tools, technologies and practices system architects need to know about for successfully scaling web applications.

While MySQL is an important product in many environments, it is only one component for a successful technology stack and for many organizations is one of several products that manage your data.

My presentation was on the common MySQL scalability mistakes and how to avoid them. This is a problem/solution approach and is a companion talk with Successful MySQL Scalability which describes design for successfully scalability …

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Understanding Innodb Transaction Isolation

The MySQL Innodb storage engine provides ACID compliance, the ‘I’ being isolation. There are four states defined in MySQL with the tx_isolation system variable, READ-UNCOMMITTED, READ-COMMITTED, REPEATABLE-READ and SERIALIZABLE.

Generally MySQL installations do not modify the default value of tx_isolation = REPEATABLE-READ, however I have seen with a number of clients the default value has been changed to READ-COMMITTED. When I attempt to identify the reason why, I have always received the same reason. Oracle uses a default transaction isolation of READ-COMMITTED. See how Oracle Manages Data …

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Understanding InnoDB MVCC

Multi versioning concurrency control (MVCC) is a database design theory that enables relational databases to support concurrency, or more simply multiple user access to common data in your database.

In MySQL the InnoDB storage engine provides MVCC, row-level locking, full ACID compliance as well as other features.

In my understanding of database theory, access to modify independent sections of unique data (e.g. UPDATE) under MVCC should fully support concurrency. I have however experienced a level of exclusive locking under Innodb.

I wanted to clearly document this situation so I could then seek the advice of the guru’s in InnoDB Internals such as Mark Callaghan, Percona and the Innodb development team for example. I’m happy …

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Showing entries 11 to 20 of 24
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