MySQL Commit Size and Speed

isotopp image Kristian Köhntopp -
July 27, 2020
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When writing data to disk, for small transactions the cost of writing the commit out do disk dominates the execution time of the script. In order to show that, I wrote a little Python script.

The script creates a test table in a database and writes 10.000 rows of test data into it, in commit sizes of 1, 2, 4, …, 1024 rows.

$ ./ --help

  Test commit sizes.

  --help  Show this message and exit.

  create    Create the demo table empty.
  drop      Drop the demo table
  fill      Write test records into the demo table.
  truncate  Truncate the demo table.

There is a small driver script to run the test. The driver creates the table, truncates it and will then run the fill command of the script over and over again, with growing commit-sizes. All powers of 2 from 0 to 10 are being tried with 10.000 rows of test data.

The test system has a very old SSD (Crucial M4-CT512M4SSD2) as a data directory.

$ cat
#! /bin/bash --

./ drop
./ create
for i in {0..10}
        csize=$((2 ** $i))
        ./ truncate
        echo "./ fill --size=10000 --commit-size=$csize"
        time ./ fill --size=10000 --commit-size=$csize

The result of the test run is as follows shown, and since at point 10.000 rows were not enough, I later added a second test run with 100.000 rows and step sizes of 5..14 (32 rows to 16384 rows).

Script runtime as a function of commit batch size. x-Axis is logarithmic, y-axis is linear.

Txn Sz Runtime Rows/s Runtime Rows/s
0 127 78
1 68.7 145
2 38.4 260
3 20.93 477
4 12.03 831
5 7.04 1420 71.01 1409
6 4.38 2283 46.40 2156
7 2.92 3412 30.13 3319
8 2.24 4464 20.94 4776
9 1.77 5649 17.51 5712
10 1.58 6329 15.35 6515
11 14.16 7063
12 13.63 7337
13 13.59 7359
14 13.26 7542

Raw data from the test runs.

We observe: Going from single row commits to 16 row commits, we get a 10-fold speed increase, and going to 1024 row commits, we are seeing another factor of 10. After that, there is hardly any improvement for our chosen record size and hardware.


A transaction size of 1000 rows or larger does not seem to improve write speed (i.e. in a data load).

From 1 row/commit to 16 rows/commit, we see an increase of an order of magnitude, and another order of magnitude can be had by going from 16 rows/commit to 1024 rows/commit.

The script

$ cat requirements.txt
$ python3 -mvenv venv
$ source venv/bin/activate
(venv) $ pip install --upgrade pip
(venv) $ pip install wheel
(venv) $ pip install -r requirements.txt
(venv) $ pip freeze -r requirements.txt > requirements-frozen.txt


#! /usr/bin/env python3

import sys
import random
import string

import click
import MySQLdb
import MySQLdb.cursors

from pprint import pprint

db_config = dict(

sql_drop_table = "drop table %s"

sql_create_table = """create table %s (
    id serial,
    data varbinary(255) not null

sql_truncate_table = "truncate table %s"

sql_insert_into = 'insert into %s ( id, data) values ( %d, "%s" )'

db = MySQLdb.connect(**db_config)"Test commit sizes.")
def sql():

@click.option("--name", default="demo", help="Table name to drop")
def drop(name):
    """ Drop the demo table """
    cmd = sql_drop_table % name

        c = db.cursor()
        click.echo(f'Table "{name}" dropped.')
    except MySQLdb.OperationalError as e:
        click.echo(f'Table "{name}" did not exist.')

@click.option("--name", default="demo", help="Table name to create")
def create(name):
    """ Create the demo table empty. """
    cmd = sql_create_table % name

        c = db.cursor()
        click.echo(f'Table "{name}" created.')
    except MySQLdb.OperationalError as e:
        click.echo(f'Table "{name}" did already exist')

@click.option("--name", default="demo", help="Table name to insert into")
@click.option("--size", default=1000000, help="Number of rows to create in total")
@click.option("--commit-size", default=1000, help="Commit batch size")
@click.option("--verbose/--no-verbose", default=False, help="Log each commit?")
def fill(name, size, commit_size, verbose):
    """ Write test records into the demo table. """
    for i in range(0, size):
        str = "".join(
            random.choice(string.ascii_uppercase + string.digits) for _ in range(20)
        cmd = sql_insert_into % (name, i + 1, str)

        c = db.cursor()
        except MySQLdb.Error as e:
            click.echo(f"MySQL Error: {e}")

        if i % commit_size == 0:
            if verbose:
                print(f"Commit at {i}...")


@click.option("--name", default="demo", help="Table name to truncate here")
def truncate(name):
    """ Truncate the demo table. """
    cmd = sql_truncate_table % name

        c = db.cursor()
        click.echo(f'Table "{name}" truncated.')
    except MySQL.OperationalError as e:
        click.echo(f'Table "{name}" does not exist.')