ext4 fs_options are ignored when using devmapper

[ksyblast]

Hello!

I am testing devmapper with K8s, containerd version v1.7.7.
OS: Centos Stream 8

I would like to mount the filesystem with a discard option however it does not work.

That's the output depicting the problem:

09:00+00 root@worker1:~# ctr i list
REF TYPE DIGEST SIZE PLATFORMS LABELS
09:00+00 root@worker1:~# ctr c list
CONTAINER    IMAGE    RUNTIME    
09:00+00 root@worker1:~# grep -A 5 io.containerd.snapshotter.v1.devmapper /etc/containerd/config.toml 
  [plugins."io.containerd.snapshotter.v1.devmapper"]
    root_path = "/var/lib/containerd/io.containerd.snapshotter.v1.devmapper"
    pool_name = "lvmvgthin-lvmvgthin_dmthin"
    base_image_size = "15GB"
    fs_type = "ext4"
    fs_options = "discard"
  [plugins."io.containerd.grpc.v1.cri"]
09:00+00 root@worker1:~# ctr images pull --snapshotter devmapper docker.io/library/ubuntu:latest
docker.io/library/ubuntu:latest:                                                  resolved       |++++++++++++++++++++++++++++++++++++++|
index-sha256:2b7412e6465c3c7fc5bb21d3e6f1917c167358449fecac8176c6e496e5c1f05f:    done           |++++++++++++++++++++++++++++++++++++++|
manifest-sha256:c9cf959fd83770dfdefd8fb42cfef0761432af36a764c077aed54bbc5bb25368: done           |++++++++++++++++++++++++++++++++++++++|
layer-sha256:aece8493d3972efa43bfd4ee3cdba659c0f787f8f59c82fb3e48c87cbb22a12e:    done           |++++++++++++++++++++++++++++++++++++++|
config-sha256:e4c58958181a5925816faa528ce959e487632f4cfd192f8132f71b32df2744b4:   done           |++++++++++++++++++++++++++++++++++++++|         elapsed: 1.2 s                                                                    total:   0.0 B (0.0 B/s)                                         
unpacking linux/amd64 sha256:2b7412e6465c3c7fc5bb21d3e6f1917c167358449fecac8176c6e496e5c1f05f...
done: 1.888068866s  
09:01+00 root@worker1:~# ctr   run   --snapshotter devmapper docker.io/library/ubuntu:latest ubuntu
mount
/dev/mapper/lvmvgthin-lvmvgthin_dmthin-snap-340 on / type ext4 (rw,relatime,seclabel,stripe=16)

Any help is appreciated

[Kern--]

This was discussed on slack (https://cloud-native.slack.com/archives/C4RJZ9Z6Y/p1698388757748389), but I want to add it here too in case anyone else runs into this.

fs_options are passed to mkfs, but not mount. In this case, discard means that mkfs should discard blocks as it's formatting the thin device. See here for the mount options that are used: https://github.com/containerd/containerd/blob/main/snapshots/devmapper/snapshotter.go#L510-L529

Adding mount options is possible, but would require changes to the snapshotter. This would give you the ability to reclaim some disk space while containers are running.

There is also discard_blocks which will perform a discard when the thin devices are removed to reclaim the space used by the containers when they exit. Just a note, this is not compatible with async_remove.

[ksyblast]

Now it's clear. Thank you very much for the answer