Swap mode fails on filesystems that don't support O_DIRECT.

[Mysticial]

This bug has been around for ages, but was only discovered now.

In Swap Mode on Linux, all computation disk access is done using O_DIRECT to bypass the system cache. If y-cruncher fails to create or open a file with O_DIRECT, it panics and bails with error 22.

The problem is that not all filesystems support O_DIRECT. This is the case for ext4 if journaling is enabled. It's been years since I've tested this on Linux, so I have no idea when this started happening by default on Ubuntu.

The current work-around is to disable journaling on all drives that will be used for swap files.

This will be fixed in v0.7.6 with fall-back logic along with a warning. But the fix will not be backported to v0.7.5 since it conflicts with on-going refactors.