How to keep the kernel from accessing the journal on an ext4 partition?
I hope that this is not a duplicate question. I have seen several similar questions, where the answer was to blacklist the respective device or partition. But in my case, I can’t do that (see below). Having said this:
On a debian buster x64 host, I have created a VM (based on QEMU). The VM runs on a block device partition, let’s say
/dev/sdc1. I have installed the debian system on that partition basically like that (some steps omitted):
#> mkfs.ext4 -j /dev/sdc1 #> mount /dev/sdc1 /mnt/target #> debootstrap ... bullseye /mnt/target
Then I bind-mounted the necessary directories (
/sys etc.), chrooted into
/mnt/target, completed the guest OS installation and booted the VM.
The VM first started without issues. But with every VM reboot, the VM got more problems, which I was repairing at the
initramfs prompts, until repairing was not possible any more because obviously the
ext4 file system had been damaged.
Because I originally thought that I had done something wrong, e.g. forgot to unmount the
ext4 partition before starting the VM, I repeated the whole installation from scratch multiple times. The result was the same in every case: After a few restarts, the
ext4 file system was so damaged that I couldn’t repair it.
Accidentally, I have found the reason for this, but have no idea how to solve the problem. I noticed that
e2fsck refused to operate on that partition, claiming that is was in use although it was not mounted and the VM was not running. Further investigation showed that there existed a kernel thread
That means that the host kernel accesses the journal on that partition / file system. When I start the VM, the guest kernel of course does the same. I am nearly sure that the corruption of the file system is due to both kernels accessing the file system, notably the journal, at the same time.
How can I solve the problem?
I cannot blacklist the respective disk or the respective partition on the host, because I need to mount them there to prepare or complete the guest OS installation in a chroot. On the other hand, it doesn’t seem possible to tell the host kernel to release the journal as soon as the VM starts.
I have installed a lot of VMs in the past years exactly the same way, but did not turn on the journal when creating their
ext4 file system. Consequently, I didn’t have that issue with those VMs.
In case it is relevant, when mounting the partition and chrooting into it to complete the guest OS installation, I use the following commands:
cd /mnt mkdir target mount /dev/sdc1 target mount --rbind /dev target/dev mount --make-rslave target/dev mount --rbind /proc target/proc mount --make-rslave target/proc mount --rbind /sys target/sys mount --make-rslave target/sys LANG=C.UTF-8 chroot target /bin/bash --login
When unmounting, I just do
umount -R target
umount command does not report any error.
-o norecovery to
mount, you could mount the filesystem without making use of the journal at all.
Man page for mount, ext3 section:
Don’t load the journal on mounting. Note that if the filesystem was not unmounted cleanly, skipping the journal replay will lead to the filesystem containing inconsistencies that can lead to any number of problems.