Once upon a time, it would have been considered madness to even attempt to create 30,000 tables in InnoDB. That time is now a memory. We have customers with a lot more tables than a mere 30,000. There have historically been no tests for anything near this many tables in the MySQL test suite.
So, in fleshing out the test cases for this and innodb_dict_size_limit I was left with the not so awesome task of making the test case run in remotely reasonable time. The test case itself is pretty simple, a simple loop in the not at all exciting mysqltest language that will create 30,000 identical tables, insert a row into each of them and then drop them.
Establishing the ground rules: I do not care about durability. This is a test case, not a production system holding important data which means I can lie, cheat and steal to get performance.
The simplest way is to use libeatmydata. This is a small shared library designed to be LD_PRELOADed that disables just about every way an application can write data safely to disk. This is perfect for running a test suite for a database server as if your machine crashes halfway through a test run, you’ll just start the test run again – you are not dealing with any important data. Obviously, you should never, ever, ever use libeatmydata with anything you care about; it is called libeat-my-data for a reason.
Without libeatmydata and using the default options for the MySQL test suite, I noticed that I was only creating about 10-15 tables every second which means we’d take a very long time to create 30,000. After a bit of time, it sped up to about 20-25 per second. Of course, with the default timeout for a MySQL test (15 minutes), we quickly (well.. in 15 minutes) hit that and the test is failed for taking too long.
With libeatmydata the test takes about 77 seconds – a huge improvement.
So how could I possibly get this test to run (even with –big-test option to mysql-test-run) in reasonable time? Well… I can set some InnoDB options! I’m going to try the obvious first: innodb-flush-method, sync-frm and innodb-flush-log-at-trx-commit. There is an undocumented option for innodb-flush-method called “nosync” that is meant to not flush data to disk. Since you could hear how much syncing to disk was going on during my test run, not syncing to disk all the time would get closer to the libeatmydata performance. I also want to disable syncing of the FRM file to disk and set log flushing to happen as infrequently as possible. With these options I started to get somewhere between 25-90 CREATE TABLE per second. This gets the test execution time down to 12 minutes, so that just escapes the timeout.
I then added the options of innodb-adaptive-checkpoint=0 and flush-neighbor-pages=0 in the hope of avoiding a bunch of background flushing (which called fsync). It didn’t help.
I noticed that there was an fsync() call when extending the data file, so I tried setting a higher innodb-autoextend-increment and a larger initial size. This also did not help.
So how fast is InnoDB under all of this? Am I hitting a fundamental limitation in InnoDB?
Well…. I went and wrote a program using HailDB – which is InnoDB as a shared library that you can call using an easy to use C API.
Writing a simple test program that creates 30,000 tables in a similar InnoDB configuration as default MySQL is pretty easy (easier than writing the mysqltest language that’s for sure). After a “I’m glad this isn’t a SSD” killer amount of fsync() activity, it took a total of 14.5 minutes. Not too bad. This is less than my initial test with MySQL, probably due to not writing and syncing FRM files. If I run the same program with libeatmydata, it only takes 15-20 seconds. Clearly it’s syncing things to disk that takes all the time.
If we make the HailDB program set flush_method to nosync and flush_log_at_trx_commit=2, the execution time is only 1 minute. This is much closer to the libeatmydata time than MySQL ever got.
With HailDB you can do more than one data dictionary operation in a single transaction. So if instead of setting flush_method and flush_log_at_trx_commit I instead group the CREATE TABLE into transactions of creating 100 tables at a time, I get an execution time of 3 minutes. This is a big difference to the original 14.5 minutes.
What’s the practical applications of all of this? Not much (unless you’re writing complex test cases) but it is clear that loading large amounts of DDL could be a lot faster than it is currently (although loading the data into tables is still going to take a while too).