I've found that in recent kernel versions (5.7+) that fallocate creates
files on ext4 filesystem with holes. Here is a repo:
$ sudo fallocate -l 512MiB /mnt/swapfile
$ sudo filefrag -e /mnt/swapfile
Filesystem type is: ef53
File size of /mnt/swapfile is 536870912 (131072 blocks of 4096 bytes)
ext: logical_offset: physical_offset: length: expected: flags:
0: 0.. 63487: 34816.. 98303: 63488:
unwritten
1: 63488.. 126975: 100352.. 163839: 63488: 98304: unwritten
2: 126976.. 131071: 165888.. 169983: 4096: 163840:
last,unwritten,eof
/mnt/swapfile: 3 extents found
$ sudo chmod 0600 /mnt/swapfile
$ sudo mkswap /mnt/swapfile
$ sudo swapon /mnt/swapfile
swapon: /mnt/swapfile: swapon failed: Invalid argument
$ sudo journalctl | grep hole
Oct 15 15:42:59 hostname kernel: swapon: swapfile has holes
Oct 15 15:43:09 hostname kernel: swapon: swapfile has holes
Oct 15 15:43:23 hostname kernel: swapon: swapfile has holes
But in previous versions (4.19 verified) this operation succeeds and
swapon doesn't indicate holes in the file.
Is the behavior of creating files with fallocate on ext4 filesystems
with holes expected and by-design?
Thank you in advance for any insight!
Thomas
