Debian

I did some better testings on the same system with 850,000 unique files on a directory (3.3G of space) located on an ext4 partition and all files are already owned by "mysql" (or the target user).

It takes between 1min30s and 1min40s to run the classic "chown -R" while the "find" variant only takes around 1.8 seconds, which is significantly faster.
I did repeat the test multiple times and got a similar result every times.

Having only one file owned by a different owner (for example "debian-10.0.flag" owned by root) on the folder wont make an significant difference.

The main drawback of the "find" alternative, is that it will be slower if an important number of files in the folder are owned by a different user than "mysql" (or the target one).
This case should not be a common one as by default files are created by "mysqld" using the "mysql" user.

Using "find" and "xargs" would require to add a dependency to the "findutils" package (both files are in it) but its possible to stick to the old behavior in case the package is not present on the system.

jb-boin Is find /var/lib/mysql/ ! -uid $(id -u mysql) -print0 |xargs -0 -r chown mysql now your final suggestion or do you have improved versions of it?

How about find $mysql_datadir ! -uid $(id -u mysql) -exec chown mysql:mysql {} \; ?

Fixed in https://github.com/MariaDB/server/commit/3a6e781ea39b67fabf1a528e6a071215a65f50b0 for 10.2

Sorry for the delay but just to answer to you last question, the use of "find ... -exec" would call chown on each file wile piping to xargs would call chown with multiple arguments and thus doing less forking and being faster.

Fixed for 10.1 in Debian in https://github.com/ottok/mariadb-10.1/commit/7d52f516e41f4af9cb5ec2eb97f6f35240914302

For find, -uid $(id -u mysql) is equivalent to -user mysql.

xargs has a --max-procs=max-procs which could be useful for parallelism of the changes.