[lustre-discuss] Issues regarding corss-version mounting of MDT

[Huijun Wu]

Andreas, thanks for your reply.

The MDTs were originally created using Lustre 2.15.4. In my case, I accidentally mounted one of the MDTs with Lustre 2.16.1. The system has two MDTs in total: one remained untouched by 2.16.1, while the other was mounted once with 2.16.1.

After that, I switched back to Lustre 2.15.4 and mounted both MDTs. At that point, some files and directories appeared as “????”. It seemed that some kind of inconsistency between the states of the two MDTs caused the issue.

Later, I mounted the second MDT (the one that had not been touched before) with 2.16.1 as well, and then switched back to 2.15.4 to mount both MDTs. After this, the problem was resolved.

I’m curious about what kinds of inconsistencies between the MDTs might have led to this behavior.

Thanks in advance for your insights.


Cheers，
Huijun




         Original
         
       
From: Andreas Dilger <adilger at thelustrecollective.com>
Date: Feb 7, 2026 08:10
To: Huijun Wu <huijun18 at foxmail.com>
Cc: lustre-discuss <
>
Subject: Re: [lustre-discuss] Issues regarding corss-version mounting of MDT



       On Feb 3, 2026, at 06:01, Huijun Wu <huijun18 at foxmail.com> wrote:
> I recently encountered an issue related to cross-version mounting of an MDT.
> After an MDS crash, I rebooted the MDS and accidentally mounted an MDT that was originally created and used with Lustre 2.15.4 using Lustre 2.16.1. After this, I noticed that many files and directories were shown as “????” on the client side.
> I then ran lfsck on this MDT, and found that some directories that originally belonged to this MDT disappeared entirely.
> Any recommendations on how to analyze, recover, or mitigate this issue?

There definitely shouldn't be any issues upgrading from a 2.15 MDT to a 2.16
MDT, and this is tested regularly during normal Lustre development.

Did you have any unusual features enabled in Lustre or ldiskfs?  Was the
filesystem originally formatted with 2.15, or was it formatted with an
older version and upgraded already?  Were there error messages on the MDS
or client console when accessing the problematic files?

Cheers, Andreas
---
Andreas Dilger
Principal Lustre Architect
adilger at thelustrecollective.com
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