Overhead seems to be about 4KiB, at least using default ext4 parameters: # zfs create -V 100G -b 4096 tank/test && mkfs.ext4 -v /dev/zvol/tank/test && mount

Overhead seems to be about 4KiB, at least using default ext4 parameters: # zfs create -V 100G -b 4096 tank/test && mkfs.ext4 -v /dev/zvol/tank/test && mount /dev/zvol/tank/test /mnt && cd /mnt # df -k . Filesystem 1K-blocks Used Available Use% Mounted on /dev/zd16 102626232 24 97366944 1% /mnt # for x in `seq 1000000`; do echo $x >$x; done # create 1M tiny files # df -k . Filesystem 1K-blocks Used Available Use% Mounted on /dev/zd16 102626232 4022348 93344620 5% /mnt # bc -l (97366944-93344620)*1024/1000000 # free space diff per file 4118.859776 Plus you're wasting an inode per record - a limited resource in ext4, increasing which requires reformatting. You'd probably run out of inodes much sooner than out of space.