[lustre-discuss] ZFS OST on-disk structure

[Thomas Roth]

Hi all,

is there somewhere an explanation of the internal structure/content of an OST, in particular a ZFS-based one?

Currently, we are decomissioning some old servers. Having emptied the OSTs of all 'findable' files, there are still  differences in what "df" tells me 
about the OSTs. So  I umounted and remounted the OSTs as local ZFS filesets. Indeed there were some few OST objects that obviously had been lost from 
the MDTs. Nothing is missed by the users, so that's fine.
However, while most of the OSTs are "empty" at about 80 MB, I have a few that show 200 MB, but no orphaned objects inside.
Searching for such objects, I noticed that e.g. directories d0...d3x in /O/0/ seem to be about 3M in size, with no visible content, while the 
corresponding directories on an 80MB-OST are ~600K.

So I am curious about these interna of OSTs. I have already learned that the famous LAST_ID file does exist on ZFS but is not visible either - could 
there be more hidden stuff like that here?

Best regards,
Thomas