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This is available in AIX 7.2 TL 03
Added new option to chfs command to reclaim unused blocks.
chfs -a reclaim=[normal | fast]
There is a developer works article about how FS reclaim works, under attachments
https://www.ibm.com/developerworks/community/wikis/home?lang=en#!/wiki/Power%20Systems/page/JFS2%20filesystem%20support%20for%20storage%20space%20reclaim
This would be valuable as a filesystem mount option. Unmap and thin allocation does have a latency impact, so it would not be appropriate as a default setting.
Today, I will occasionally fill free space with nulls, but that is not always safe since it very briefly leaves the filesystem full.
Alternatively, you can chfs to reduce size, which also unmaps, then expand back. The drawback is FS lag in some instances as it consolidates or frees empty structures. It also can fail to reduce empty space due to fragmentation. Either of these is more of an occasional operation, and not a cron or realtime task.
Having a version of secure delete that overwrites with nulls would be effective, though not as performant as a scsi unmap.
I think one factor is that JFS deletes are not guaranteed to free contiguous blocks, so unmap commands at the file level may not be as useful.