For GFS1, we can't change disk layout so we borrow the "license" file
that happens to be an unused on-disk GFS1 file. There is only one per
file system, comparing to GFS2 that uses N+1 files (N is the number of
nodes in this cluster) to handle the "df" statistics. Every node keeps
its changes in memory buffer and syncs its local changes to the master
(license) file every 30 seconds. Upon unclean shutdown (or crash), the
local changes in the memory buffer will be lost. To re-sync the
correct statistics, we need to use real "df" command (that scans the
on-disk RGRP disk structures) to adjust the correct statistics. For
details, check out one of my old write-ups in:
http://people.redhat.com/wcheng/Patches/GFS/readme.gfs_fast_statfs.R4
BTW, I'm not very happy with this implementation and there are few other
ideas on the table. However, since GFS2 is imminent (I hope), will keep
the code as it is today.
-- Wendy
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