In the past it was not possible to shrink a JFS. Has that changed in the meantime, is somebody working on it, is it planned ...?
How would it be possible to replace JFS with another FS without having to backup and restore the files?
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It's not possible to shrink a JFS file system using the standard tools. Compunix provides some tools that claim to be able to shrink JFS filesystems, but there's no widespread account of their reliability or success rates.
It's generally not possible to switch filesystems in-place, unless you're changing versions of the same filesystem (ext2->ext3->ext4) or converting FAT to NTFS on certain Windows systems.
In theory, if your JFS file system doesn't take up the whole disk, and you're very lucky about where the FS is placed on the disk, you could copy files to another partition, then delete the JFS partition and grow the other partition and its filesystem (be sure to pick something that can grow, like ReiserFS or XFS).
According to this post it's only possible to shrink JFS2.
Sadly, I don't think you can migrate from JFS to JFS2 online.
If you have backups in place, you could give it a shot and try convertfs - it does something similar to the suggestion from "yesterday", but in small steps and automated. (But then again, if you have backups - isn't it easier to just recreate the fs and restore? :)
In theory it should work. (I have actually used it with success...once)