[Linux-cluster] GFS2 file system maintenance question.

Jack Duston jduston at ll.mit.edu
Mon Mar 14 22:35:30 UTC 2011

Hello folks,

I am planning to create a 2 node cluster with a GFS2 CLVM SAN.
The following Note in the RHEL6 GFS2 manual jumped out at me:

Chapter 3. Managing GFS2
Once you have created a GFS2 file system with the mkfs.gfs2 command, you 
cannot decrease the size of the file system. You can, however, increase 
the size of an existing file system with the gfs2_grow command, as 
described in Section 3.6, “Growing a File System”.

This seems to me to make a GFS2 LV un-maintainable.

What concerns me is the issue of how to remove a LUN from the GFS2 LV. 
This will be a necessity *when* there are hardware problems with a 
storage unit, End of Life/obsolescence (a la XRaid), or upgrade (replace 
1TB HDDS with 3 TB HDDs in the LUNs).

Hardware does not last forever, and manufacturers do EOL products or go 
out of business.
I had also hoped to upgrade the 1TB HDDs in our current LUNs with 3 TB 
HDDs next year.

I planned to free up enough space on the GFS2 LV to migrate data off one 
LUN. I could then decrease the GFS2 file system size, remove the LUN 
from the LV, destroy the RAID LUN, replace 1TB HDDs with 3TB HDDs, 
rebuild the RAID LUN, add the new larger LUN to the LV, increase the 
GFS2 file system size, and repeat migrating data off the next LUN.

If the above note is correct, it seems to only way to deal with a 
hardware issue, obsolescence/EOL, or upgrading components is to destroy 
the entire GFS2 file system, build a new GFS2 file system from scratch, 
and restore data from backups. This might not be too bad with a small 
SAN of 20TB, but our data will exceed 100TB and it would be good not to 
have to rebuild Rome in a day.

Can anyone confirm that GFS2 file system cannot be decreased? If so, is 
there any plan to add this capability/fix this issue in a future 
release? Is there another/better way to remove a LUN from GFS2 than what 
I considered?

Any info greatly appreciated.

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