MySQL on ZFS (on FreeBSD)

logofreebsdLots of FreeBSD users are coming to ZFS since the release of FreeBSD 10, as it's so easy to install the system on top of that powerful filesystem. ZFS default behavior and settings are perfect for a wide range of workloads and uses, but it's not exactly what you need for databases hosting.
I'm running a FreeBSD 9.x server hosting http, php, mysql, mail, and many other things, so a databases-only optimization would be counterproductive. After a good dive into experts do's & don't's here are few steps I've taken to tune my server.


If like me you've started hosting MySQL databases a long time ago (say ~15 years ago) you might have some DB using the old MyISAM engine, and some newer ones using InnoDB. Guess what. They use different block sizes, and they use different caching mechanisms.
On top of that, the block size for InnoDB is not the same when you deal with data files, and when you deal with log files.
In order to get the best block size and cache tuning possible, I've created 3 datasets: one for InnoDB data files, one for innodb logs, and one for everything else, including MyISAM databases.

Block size

The block size is engine dependent. MyISAM uses a 8k block size, InnoDB uses a 16k block size for data and 128k for logs. It's easy to setup datasets for a particular block size at creation with -o option, or after creation with zfs set. You must set the proper block size before putting any data on the dataset, otherwise pre-existing data won't use the desired block size.


MyISAM engine relies on the underlying filesystem caching mechanism, so you must ensure ZFS will cache both data and metadata (that's the default behavior). On the other hand, InnoDB uses an internal cache, so it would be a waste on memory to cache the same data into ZFS and InnoDB. On a dedicated MySQL server, the proper tuning would require to limit ARC size, and to disable data caching in ZFS. On a general-purpose server, ARC size should not be tweaked, but on InnoDB dedicated datasets it's easy to disable ZFS cache for data by setting the property primarycache to "metadata".

On MySQL's side

You must of course tell mysqld where to find data files and logs. I've set those values in my /var/db/mysql/my.cnf file:

innodb_data_home_dir = /var/db/mysql-innodb
innodb_log_group_home_dir = /var/db/mysql-innodb-logs

where /var/db/mysql-innodb and /var/db/mysql-innodb-logs are mount points for the dataset dedicated to InnoDB data files, and the dataset dedicated to InnoDB log files.

As my zpool is a mirror, I've added this setting too:


Step by step

I've followed this course of action:

1st step: mailing to users, "MySQL will be unavailable for about 5 minutes, hang on."
2nd step: backup /var/db/mysql and edit your my.cnf
3rd step: shutdown your mysql server

sudo service mysql-server stop

4th step: move /var/db/mysql to /var/db/mysql-origin
5th step: create appropriate datasets:

zfs create -o recordsize=16k -o primarycache=metadata zmirror/var/db/mysql-innodb
zfs create -o recordsize=128k -o primarycache=metadata zmirror/var/db/mysql-innodb-logs
zfs create -o recordsize=8k zmirror/var/db/mysql

6th step: move data from /var/db/mysql-origin to your new datasets

cd /var/db/
sudo mv mysql-origin/ib_logfile* mysql-innodb-logs/
sudo mv mysql-origin/ibdata1 mysql-innodb/
sudo mv mysql-origin/* mysql/

and set proper rights:

sudo chown mysql:mysql mysql-innodb-logs mysql-innodb mysql
sudo chmod o= mysql mysql-innodb-logs mysql-innodb

7th step: restart your mysql server

sudo service mysql-server start & tail -f mysql/${HOSTNAME}.err

8th step: mailing to users, "MySQL's back online maintenance duration 5 min 14 sec."

Further reading & references

MySQL Innodb ZFS Best Practices
A look at MySQL on ZFS
Optimizing MySQL performance with ZFS
ZFS for Databases

Related posts


  1. thanks!!! i've been struggling for a long time how to handle the different record size for myisam and innodb, and your article helped me!

  2. Any reason why mysql would fail to start after settings this on a debian install of mariadb? I also zfs set sync=disabled on the dataset, are there any other settings mysql wants?

    error log shows:
    2018-08-17 17:17:03 140391627457088 [ERROR] mysqld: File './ON.index' not found (Errcode: 13 "Permission denied")
    2018-08-17 17:17:03 140391627457088 [ERROR] Aborting

  3. It looks like you have a permission problem. ZFS settings shouldn't have any impact on mysqld ability to start. Have you double checked the file system rights?

  4. My understanding of MyISAM is that it has 2 caches: One for index blocks called the "key_buffer_cache". These are 1KB in size. The other is for data blocks, which it leaves to the OS to handle.

    Since 1KB is less than the likely disk block size, I would expect caching by the OS to be useful.

    I do not know where you got either the 8KB or the 128KB.

  5. This is very likely I got all the block size informations from references in my "Further reading & references" section. I'm a bit surprised that not even one of those links is working 7 years later.
    I'll try & find the corresponding copy…

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