Even when the output of EXPLAIN doesn’t show “using temporary”, a temporary file may still be used in certain cases.

That’s not to say the query needs the temporary file to actually resolve the query (like what you’d see from the need for a derived table). But rather, the temporary file I’m speaking of is due to binary logging.

In particular, you can see this easily if using InnoDB, (most commonly) row-based binary logging, and you issue a large transaction, say a large UPDATE (large meaning something larger than the size of binlog_cache_size). In this case, you’ll notice a temporary file being …

An update of MySQL from 5.0 to 5.5 on CentOS 5.5 64bit has not resulted in a good experience. The mysqld process would then crash every few minutes with the following message.

101120 8:29:27 InnoDB: Operating system error number 22 in a file operation.
InnoDB: Error number 22 means ‘Invalid argument’.
InnoDB: Some operating system error numbers are described at
InnoDB: http://dev.mysql.com/doc/refman/5.1/en/operating-system-error-codes.html
InnoDB: File name /tmpfs/#sql6cf3_5c_0.ibd
InnoDB: File operation call: ‘aio write’.
InnoDB: Cannot continue operation.

The work around was to change the tmpdir=/tmpfs (which was a 16G tmpfs filesystem) to a physical disk.

The referenced URL didn’t provide any more information of help. Unlike Bug #26662 O_DIRECT is not specified as the flush method.

Followup on Experiment: MySQL tmpdir on tmpfs, about tmpdir=/dev/shm in my.cnf (it’s not a dynamic variable that can be set at runtime). It’s working well, also confirmed by comments from others that they’ve been using it for a while.

This particular setting is Linux specific. On Solaris, the default /tmp is already on a tmpfs so that’s fine too. Brian reminded me that this tweak is also useful if you’re stuck with a 32-bit OS as you can then utilise some more memory in a practical way.

Extra useful hint from Harrison: if you are using replication, you will also want slave_load_tmpdir=/tmp on your slave (real disk which survives a restart). The issue is that with statement based binary logging, there are many events which create a file for a replicated LOAD DATA INFILE. If …

In MySQL, the tmpdir path is mainly used for disk-based sorts (if the sort_buffer_size is not enough) and disk-based temp tables. The latter cannot always be avoided even if you made tmp_table_size and max_heap_table_size quite large, since MEMORY tables don’t support TEXT/BLOB type columns, and also since you just really don’t want to run the risk of exceeding available memory by setting these things too large.

You can see how your server is doing with temporary tables, how many of those become disk tables, and also other created temporary files, by looking at SHOW GLOBAL STATUS LIKE ‘Created_tmp%’;

So why might tmpfs be better? Well, not just any tmpfs, but specifically /dev/shm. Its …