TRIM

StorageOS volumes support TRIM/Unmap by default for all uncompressed volumes. StorageOS creates uncompressed volumes by default so all of these volumes will support TRIM/Unmap calls. TRIM’ing a StorageOS volume will release space taken up by deleted blocks in the StorageOS volume blob files.

TRIM can either be called periodically via fstrim or continously using the -o discard mount option. StorageOS recommends using periodic discard.

Periodic Discard

In order to TRIM a volume you can run fstrim against the volume filesystem. The volume filesystem will be presented at the mount point for the StorageOS device.

The example below shows the effect of fstrim on a StorageOS volume mounted on /mnt.

# A StorageOS volume with some data written to it.
$ ls -ls --block-size 1 /var/lib/storageos/data/dev1/vol.211585.*.blob | awk '!/^total/ {total = total + $1}END{print total}'
8.85838e+09

# A StorageOS volume is mounted on /mnt
$ df -h /mnt
Filesystem                                                         Size  Used Avail Use% Mounted on
/var/lib/storageos/volumes/v.bbfae475-3ce3-4238-bf33-cfe88256d813  4.9G  520M  4.1G  12% /mnt

# Delete a file from the volume, sync all filesystems to ensure data has been written and then fstrim the volume filesystem
$ rm /mnt/test; sync; fstrim /mnt

# Observe that the space in the blob files, and thus space in the backend filesystem has been reclaimed.
$ ls -ls --block-size 1 /var/lib/storageos/data/dev1/vol.211585.*.blob | awk '!/^total/ {total = total + $1}END{print total}'
469700608

Automating periodic discard

Discard can be automated by running fstrim against mounted StorageOS volumes. When running in Kubernetes the Kubelet is responsible for mounting volumes into pods so the mount endpoints for pods are accessible under /var/lib/kubelet/pods. StorageOS volume mounts appear as mounts from /var/lib/storageos/volumes/v.${DEPLOYMENT_ID} on /var/lib/kubelet/pods/${POD_UID}/volumes/kubernetes.io~csi/${PV_ID}/mount.

[email protected]:/# mount | awk '/storageos\/volumes\/v.*/'
/var/lib/storageos/volumes/v.4364e143-865e-45d7-a4a5-e0d964d9e200 on /var/lib/kubelet/pods/29ec4774-71d6-4ba3-b275-f80b47e9f6af/volumes/kubernetes.io~csi/pvc-357a9baa-7b74-49db-8d63-c540b5129ad8/mount type ext4 (rw,relatime,stripe=32)
/var/lib/storageos/volumes/v.8dc449a4-1fb5-43d8-86d0-e18916a85c18 on /var/lib/kubelet/pods/6bb92a8f-afb5-48c8-837c-56f45e15e5c4/volumes/kubernetes.io~csi/pvc-2d30a2ba-2663-4036-bb1e-e795f226d6f3/mount type ext4 (rw,relatime,stripe=32)

[email protected]:/# mount | awk '/storageos\/volumes\/v.*/ {print $3}'
/var/lib/kubelet/pods/29ec4774-71d6-4ba3-b275-f80b47e9f6af/volumes/kubernetes.io~csi/pvc-357a9baa-7b74-49db-8d63-c540b5129ad8/mount
/var/lib/kubelet/pods/6bb92a8f-afb5-48c8-837c-56f45e15e5c4/volumes/kubernetes.io~csi/pvc-2d30a2ba-2663-4036-bb1e-e795f226d6f3/mount

With this information in mind it is therefore possible to use Kubernetes tools to automate periodic discard. The SYS_ADMIN capability is required to run fstrim and propagation of mounts from the host to the container will allow any new StorageOS mounts to be picked up by the unmap pod.

The naïve example below shows how a pod could be used to run fstrim against StorageOS volumes mounted on the same node that the pod is scheduled on.

apiVersion: v1
kind: Pod
metadata:
  name: discard
spec:
  containers:
    - name: fstrim
...
      command: ["/bin/bash"]
      args: ["-c","for mount in $(mount | awk '/storageos\\/volumes\\/v.*/ {print $3}'); do if [ -d ${mount}  ]; then fstrim -v ${mount}; fi; sleep 5; done"]
      securityContext:
        capabilities:
          add:
            - SYS_ADMIN
      volumeMounts:
      - mountPath: /var/lib/kubelet
        mountPropagation: HostToContainer
        name: kubelet-dir
  volumes:
  - hostPath:
      path: /var/lib/kubelet
      type: Directory
    name: kubelet-dir

Alternatively a cronjob on the node itself could be used to fstrim mounted StorageOS volumes using similar logic.

N.B. TRIM can be an I/O intensive operation so care should be taken when running fstrim against multiple volumes at once.

Continuous discard

StorageOS volumes can be mounted using the discard option which will automatically send TRIM commands when blocks are removed. Caution should be used enabling this option as testing has shown that volumes with a lot of churn can experience performance degradation. The pathological case being a volume that is continuously filled with small files that are then all deleted, repeatedly. The RHEL documentation also recommends doing perodic discards.

The discard option can be enabled as a StorageClass or PersistentVolume option. Enabling discard as a StorageClass option will result in all volumes provisioned with that StorageClass being mounted with discard whereas setting it as a PersistentVolume option sets it on a per volume basis.

The StorageClass below would provision xfs volumes with the discard option enabled by default.

apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: storageos-discard
provisioner: csi.storageos.com
allowVolumeExpansion: true
parameters:
...
  csi.storage.k8s.io/fstype: xfs
mountOptions:
  - discard

You can also edit an existing PersistentVolume to add discard to .spec.mountOptions, this ensures that the next time the volume is mounted it will use -o discard.

apiVersion: v1
kind: PersistentVolume
...
spec:
...
  mountOptions:
  - discard
  volumeMode: Filesystem