gp_disk_free simply runs the df command on each segment. It reports the free size in kilobytes.
You have 4 segments on 2 segment servers. Each segment is using a different disk; that is a good strategy.
Segment 2, which is running on sdw2 and using /dev/md126 is the segment that is low on disk. It has only 1.1 MB.
If I assume that nothing other than the database is using disk space, then there is a problem with skew. Some of your tables have distribution policies that are causing many more rows to be placed on segment 2 than on the other segments.
You can investigate this with something like this:
select gp_segment_id, count(1) from <your_table_name> group by 1 order by 1 ;
The solution is to change your distribution policy. Either choose a different column to place in the "distributed by" clause or use the "distributed randomly" clause.
If it isn't a skew problem, then look at what is consuming disk space on sdw2.
Leonard Walstad
Pivotal
www.gopivotal.com