Whilst working with a client recently I noticed a large number of temporary tables being created on disk.
12345 show global status like 'Created_tmp%'| Created_tmp_disk_tables | 91970 || Created_tmp_files | 19624 || Created_tmp_tables | 1617031 |
Looking at a 60 second sample showed there was a steady flow of queries creating disk based tables.
12345 mysqladmin ext -ri60| Created_tmp_disk_tables | 74 || Created_tmp_files | 3 || Created_tmp_tables | 357 |
Luckily this client was running the Percona patched version of MySQL.
The microslow patch adds a very useful feature, the ability to log queries by execution plan.
1 | <span style="color: #38acec;">log_slow_filter=name</span> |
Log only the queries that followed certain execution plan. Multiple flags allowed in a comma-separated string. [qc_miss, full_scan, full_join, tmp_table, tmp_table_on_disk, filesort, filesort_on_disk]
So now all I had to do was set the filter to “tmp_table_on_disk,filesort_on_disk†and I would get only those which use on-disk temporary storage for intermediate results.
1 SET GLOBAL log_slow_filter:= “tmp_table_on_disk,filesort_on_diskâ€;
Wait a couple of minutes and then set the filter to an empty string to stop collection.
1 SET GLOBAL log_slow_filter:="";
Then it was just a question of examining the slow query log and fixing the queries.
More information about the patches is available here https://www.percona.com/percona-lab.html
Thanks Ewen,
Nice summary of this not so commonly used option. There is also an other way to do it – by simply logging queries with query plans and post filtering the log file.
Currently mysqlsla isn’t very er, intuitive for post-filtering 🙂 I took the same approach Ewen did and found it easier.
Hi,
Have you guys done any benchmark on I/O performance with msl patch ?
I will be interesting to know the results if that is already done.
🙂
Hareesh