Perhaps you could argue that someone using data=journal mode doesn't care about write performance, but that would not necessarily be the case. It would be really nice if I could use ext4 with data journaling AND get delayed allocation. I'm just mentioning this because I'm probably not the only one out there with this feeling. In fact, I know I'm not because I've seen the question asked in forums. I suspect it might be nontrivial to modify the journaling system so that you can write data to the journal that has not yet been allocated a location on the disk, but it seems to me that's basically what you'd have to do. (Otherwise it wouldn't be DELAYED allocation, and we need data to hit the journal before it hits the fs proper.) Data would be written to the journal, then when it's going to be committed to the fs, it would be allocated (and that fact added to the journal?). If the system goes down, and a journal replay is required, then the data would be allocated space at that time. One thing to be concerned about is having an allocation happen, the data not written to where it's allocated, and the allocation not logged in the journal--on replay, you might allocate yet more space for the data, and you'd need some means to recover the first allocation that's now bogus. Thanks.
Why was this marked obsolete? According to this documentation on kernel.org, ext4 still does not support delayed allocation in data=journal mode: http://kernel.org/doc/Documentation/filesystems/ext4.txt I've also spent some time googling this, and I haven't been able to find any announcement that this has been fixed.
Because its 2 years old and we don't use bugzilla to track feature wishlists If you want it supported send patches.