[lustre-discuss] Replacing ldiskfs MDT with larger disk

[Jesse Stroik]

Hi everyone,

One of our lustre file systems outgrew its MDT and the original scope of 
its operation. This one is still running ldiskfs on the MDT. Here's our 
setup and restrictions:

- centos 6 / lustre 2.8
- ldiskfs MDT
- minimal downtime allowed, but the FS can be read-only for a while.

The MDT itself, set up with -i 1024, needs both more space and available 
inodes. Its purpose changed in scope and we'd now like the performance 
benefits of getting off of spinning media as well.

We need a new files system instead of expanding the existing ldiskfs 
because we need more inodes.

I think my options are (1) a file level backup and recovery or direct 
copy onto the new file system or (2) add a new MDT to the system and 
assign all directories under the root to it, then lfs_migrate everything 
on the file system thereafter.

Is there a disadvantage to the DNE approach other than the fact that we 
have to keep the original spinning-disk MDT around to service the root 
of the FS?

If we had to do option 1, we'd want to remount the current MDT read only 
and continue using it while we were preparing new MDT. When I searched, 
I couldn't find anything that seemed definitive about ensuring no 
changes to an ldiskfs MDT during operation and I don't want to assume i 
can simply remount it read only.

Thanks,
Jesse Stroik

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