Hi,
we consistently see slow unmounts (~6–8s) on ext4 after heavy buffered writes
(e.g., dd ~30 GiB) that grow the page cache. Same test on XFS unmounts <1s on
the same hardware, but we must stay on ext4.
Env:
- Kernel: 6.6.36 (Yocto-based)
- Device: SSD via mdraid (/dev/md0p1)
- Mount: ext4 on /mnt/disk (defaults)
Repro:
dd if=/dev/zero of=/mnt/disk/big.bin bs=1M count=30720 status=progress
sync -f /mnt/disk
time umount /mnt/disk # ext4: ~6–8s
Observations:
- Dirty/Writeback are ~0 before unmount.
- `fsfreeze -f /mnt/disk` immediately before `umount` makes unmount very fast.
- `mount -o remount,ro` before `umount` does NOT improve unmount time.
Questions:
1) Is this unmount time expected on ext4 with a large *clean* page cache?
2) Why is XFS unmount much faster here—any ext4-side reasons or regressions?
3) Is `fsfreeze -f` → `umount` acceptable/recommended for shutdown on ext4?
Any ext4/VFS options or per-filesystem methods to reduce unmount time?
Looking forward for an answer!
Thx,
Rafael
