mysql-server-5.7 fails to install on box with separate /var/lib/mysql partition

Thanks, we should fix this. Separately from this though I'd recommend creating a directory inside your filesystem and using that, because we have /var/lib/mysql-keyring and /var/lib/mysql-files now and you probably want those in that filesystem too.

Ah cool, thanks for the tip Robie!

It's a limitation of the --initialize option used to create the database that it's very picky about what can be in datadir. It will ignore anything starting with a ., but for anything else it will fail (unless specified with --ignore-db-dir).
That's the reason we assume that if there's anything in datadir we shouldn't run database initialization.

ignore_db_dirs=lost+found by default maybe?

Part of the problem is that any directory inside datadir will be treated as a schema, which could produce some pretty odd results in this case.
While changing mysqld --initialize to ignore this directory would fix the failure and be safe, to avoid having it be read as a schema we'd need to add the same to the default server config, which I'm a bit more reluctant to do, though granted it's not very likely that people will want a schema named lost+found :)
As a first step we could add a check to detect the situation (since it does seem to be a fairly common use case), and add a warning explaining the issue.

sounds root regarding mysqld --initialize.

slightly laxer option for
mountpoint -q ${datadir} && cp my.lost_found.cnf /etc/mysql/cnf.d/....

to be really prudent you could see if lost+found is empty or contains only whatever filename extX "recovers" files to.

I have arrived to this issue as I have trouble installing mysql with data on a Logical Volume.

Traditionally I would just mount a Logical Volume on /var/lib/mysql prior to installing mysql-server. That would setup the database from the start on the volume.

I am reading this thread and I see that there are now 3 directories with data, plus trying to avoid lost+found.

What is the process for installing mysql-server with another data filesystem/volume?

is it possible to create a config file prior to installing mysql-server so that it will get installed using the new filesystem?

What should the process be?

Unfortunately, we do not have a doc explaining how to achieve that. I am also not aware of a way to "preseed" the mysql-server package to do what you want. What you could try to do in order to avoid lost+found is to make use of a symlink, for instance mount it into /mysql and then create a symlink from /var/lib/mysql to /mysql/data.