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Displaying posts with tag: deadlocks (reset)
Avoiding Deadlocks in Galera - Set up HAProxy for single-node writes and multi-node reads

September 17, 2013 By Severalnines

Galera cluster has known limitations, one of them is that it uses cluster-wide optimistic locking. This may cause some transactions to rollback. With an increasing number of writeable masters, the transaction rollback rate may increase, especially if there is write contention on the same dataset. It is of course possible to retry the transaction and perhaps it will COMMIT in the retries, but this will add to the transaction latency. However, some designs are deadlock prone, e.g sequence tables. In this blog we present how you can minimize the risk for deadlocks due to the design of Galera. 


Test Case


Here is a simple test case. We have a table that contains a column and a row that hold a number and is regularly updated to provide …

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Bulk insert into tables in sorted order to avoid deadlocks

Shard-Query inserts data into a “coordinator” table when answering queries.   When there is a GROUP BY on the original query, the coordinator table contains a UNIQUE KEY over the GROUP BY attributes.   Shard-Query uses INSERT .. ON DUPLICATE KEY UPDATE in combination with bulk insert (insert into … values (),(),() ) when inserting into the table.

For what would normally be efficiency sake, Shard-Query sends queries to the shards using ORDER BY NULL which disables the filesort operation. Of course, this often results in the rows being sent back from the shards in random order.

Because the results are in random order, the bulk insertion that the worker does into the coordinator table can deadlock with other worker threads when using InnoDB or TokuDB as the coordinator table. Right now I’ve just been using MyISAM for the coordinator table, which serializes queries at the bulk insert stage.  Having to insert the …

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MySQL 5.5's new features

The recently released MySQL 5.6 gets a lot of attention, but for those who are still on 5.5 there is also good news: There are two new features in 5.5.

The first feature is that there are more INFORMATION_SCHEMA tables for InnoDB. This means that it's possible to 'see' what's in the buffer pool. It also makes it possible to get more information about the LRU list.

From the 5.5.28 changelog:
InnoDB: Certain information_schema tables originally introduced in MySQL 5.6 are now also available in MySQL 5.5 and MySQL 5.1: INNODB_BUFFER_PAGE, INNODB_BUFFER_PAGE_LRU, and INNODB_BUFFER_POOL_STATS. (Bug #13113026)

This is in the "Bugs Fixed" section instead of the "Functionality Added or Changed" section, which is a bit weird in my opinion.

The second feature is a variable which makes it …

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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.

Archive strategies for OLTP servers, Part 3

In the first two articles in this series, I discussed archiving basics, relationships and dependencies, and specific archiving techniques for online transaction processing (OLTP) database servers. This article covers how to move the data from the OLTP source to the archive destination, what the archive destination might look like, and how to un-archive data. If you can un-archive easily and reliably, a whole new world of possibilities opens up.

Showing entries 1 to 5