MySQL: Can't create/write to file '/tmp/#sql_3c6_0.MYI' (Errcode: 2) - What does it even mean?

Question

For some reason my production DB decided to spew out this message. All application calls fail to the DB with the error:

PreparedStatementCallback; SQL [ /*lo         


        

Answer 1

The filename looks like a temporary table created by a query in MySQL. These files are often very short-lived, they're created during one specific query and cleaned up immediately afterwards.

Yet they can get very large, depending on the amount of data the query needs to process in a temp table. Or you may have multiple concurrent queries creating temp tables, and if enough of these queries run at the same time, they can exhaust disk space.

I do MySQL consulting, and I helped a customer who had intermittent disk full errors on his root partition, even though every time he looked, he had about 6GB free. After we examined his query logs, we discovered that he sometimes had four or more queries running concurrently, each creating a 1.5GB temp table in /tmp, which was on his root partition. Boom!

Solutions I gave him:

• Increase the MySQL config variables tmp_table_size and max_heap_table_size so MySQL can create really large temp tables in memory. But it's not a good idea to allow MySQL to create 1.5GB temp tables in memory, because there's no way to limit how many of these are created concurrently. You can exhaust your memory pretty quickly this way.

• Set the MySQL config variable tmpdir to a directory on another disk partition with more space.

• Figure out which of your queries is creating such big temp tables, and optimize the query. For example, use indexes to help that query reduce its scan to a smaller slice of the table. Or else archive some of the data in the tale so the query doesn't have so many rows to scan.