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In the man ext4(5) we can read the following:

Warning: The bigalloc feature is still under development, and may not be fully supported with your kernel or may have various bugs. Please see the web page http://ext4.wiki.kernel.org/index.php/Bigalloc for details. May clash with delayed allocation (see nodelalloc mount option).

According to the link above, the info is dated back to 2013, which is a little bit ancient.

What's the current status of the feature? Is it safe to use bigalloc on several TiB hard disks where only big files will be stored?

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    Best to ask this directly on the [email protected] list, and update here once you get a response. Commented Jul 22, 2021 at 14:49
  • I'll try to do that. Commented Jul 23, 2021 at 15:30
  • I asked (marc.info/?l=linux-ext4&m=162705422926698&w=2), but I don't think that someone ever is going to respond to that message... Commented Jul 25, 2021 at 9:48

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There was actually still a bug with bigalloc until Kernel 4.20. If your kernel is not up-to-date you may experience this problem: Bug 151491 - free space lossage on busy system with bigalloc enabled and 128KB cluster:

file system with bigalloc enabled and 128KB cluster size with a large number of 2MB files being created/overwritten/deleted loses usable space.

Running du and df gives wildly different usage with df showing much more usage than du. lsof shows no phantom open files. Using dd to fill the file system shows that df's version of free space is operative, but unmounting and remounting the file system returns the free space. There is no difference between df and du usage after remount.

The fault does not seem to be present in the 4.7 kernel (or it takes a lot more activity for it to show up).

However, I don't know if this is the only problem or if there are maybe others.

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