There is an mdt.*.md_stats file for each MDT, but every other device type has just a "<type>.*.stats" file to report statistics. The md_stats file format is the same as other stats files so there is no good reason to use the same name. There was historically an "obd stats" file for the MDT, but that no longer exists.

On the client, there are "mdc.*.md_stats" and "lmv.*.md_stats" files alongside the "*.*.stats" files, which partly contain duplicate statistics, and partly different statistics. There doesn't seem to be a particularly good reason for the separation. There is also "mdc.*.mdc_stats" which contains "lockless_read_bytes" and "lockless_write_bytes", which I don't think is very useful today.