Minimal filesystem documentation

[Doooooo0o]

Describe the bug
Using loki in production for a while on a ext4 filesystem without dir_index, encountering a bug like described in:

• Loki (in Docker) reports "no space left on device" but there's plenty of space/inodes #1502 (comment)
• Loki (in Docker) reports "no space left on device" but there's plenty of space/inodes #1502 (comment)
• and Loki (in Docker) reports "no space left on device" but there's plenty of space/inodes #1502 (comment) (and I think others :-))

To Reproduce
Steps to reproduce the behavior:

1. Started Loki
2. Started Promtail
3. wait for a few hours/days to fill up your directory index space.
4. cry

Expected behavior
Since it seems to be a requirement, it should be explicit in Loki's documentation that dir_index should be enabled.

Environment:

• Infrastructure: bare metal + local storage and bare metal + minio remote storage.

[stale]

This issue has been automatically marked as stale because it has not had any activity in the past 30 days. It will be closed in 7 days if no further activity occurs. Thank you for your contributions.

[pettai]

Moving away from a flat single directory is the correct solution
#364

[Doooooo0o]

@pettai indeed, but we still need to know its limitations and requirements.