Summary:

When using Proxmox VE 9.2.3 with local Btrfs storage, LXC containers whose ext4 root filesystem is created with the `mmp` feature exhibit an approximately 43 second delay on the first mount/start after rolling back to a snapshot.

Removing the `mmp` feature from the container's ext4 filesystem eliminates the delay completely.

The delay appears to correspond to ext4's Multiple Mount Protection (MMP) timeout.

Environment:

Proxmox VE: 9.2.3
Kernel: 7.0.12-1-pve
Storage backend: local-btrfs
Root filesystem: Btrfs (installed using the automated answer file)
Container template: debian-13-standard_13.1-2_amd64.tar.zst

Command used for container creation:

```
pct create 111 local-btrfs:vztmpl/debian-13-standard_13.1-2_amd64.tar.zst \
--hostname hermes \
--unprivileged 1 \
--cores 2 --memory 2048 --swap 512 \
--rootfs local-btrfs:16 \
--net0 name=eth0,bridge=vmbr0,ip=192.168.1.111/24,gw=192.168.1.1 \
--features nesting=1 \
--onboot 1 \
--ssh-public-keys /root/ssh_key.pub
```

Reproduction:

1. Create container.
2. Start container.
3. Create snapshot.
4. Modify any file inside the container.
5. Roll back to the snapshot.
6. Start or mount the container.

Example:

```
pct snapshot 111 test

pct enter 111
touch hello
exit

pct rollback 111 test

time pct start 111
```

or

```
time pct mount 111
```

---

Observed behavior:

The first mount/start after rollback consistently takes approximately:

```
43-44 seconds
```

Subsequent starts are immediate.

Boot inside the container is normal:

```
systemd-analyze
Startup finished in ~800 ms
```

The delay occurs before userspace starts.

Investigation:

The delay can be reproduced outside Proxmox by mounting the container filesystem manually.

```
LOOP=$(losetup -Pf --show $(pvesm path local-btrfs:111/vm-111-disk-0.raw))

time mount "$LOOP" /mnt/test
```

Result:

```
~43 seconds
```

Therefore the delay is not caused by LXC startup itself.

e2fsck output:

Running:

```
e2fsck -fn "$LOOP"
```

produces:

```
MMP interval is 10 seconds and total wait time is 42 seconds.
Please wait...
```

which closely matches the observed mount delay.

ext4 features:

The filesystem created by Proxmox contains:

```
Filesystem features:
...
mmp
...
```

Proxmox source:

The installed Proxmox source contains:

```perl
'mkfs.ext4', '-O', 'mmp', '-E', "root_owner=$root_uid:$root_gid", $dev,
```

located in:

```
/usr/share/perl5/PVE/LXC.pm
```

Searching the installed Perl sources shows this is the only reference to `mmp`.

Experiment:

After creating a fresh container, before taking any snapshots, I removed the MMP feature:

```
LOOP=$(losetup -Pf --show $(pvesm path local-btrfs:111/vm-111-disk-0.raw))

tune2fs -O ^mmp "$LOOP"

e2fsck -fy "$LOOP"

losetup -d "$LOOP"
```

Verification:

```
tune2fs -l
```

shows the `mmp` feature is no longer present.

I then:

1. Started the container.
2. Created a snapshot.
3. Modified files.
4. Rolled back.
5. Started the container again.

Result:

```
time pct start 111

real 0m1.773s
```

The ~43 second delay disappeared completely.

Question:

Is enabling `-O mmp` for every ext4 LXC root filesystem intentional?

If so, is the 42-43 second mount delay after snapshot rollback expected?

If not, should `mmp` perhaps be omitted for loop-backed local LXC root filesystems, where multiple concurrent mounts cannot normally occur?

Thank you all
Akash