Examining GrootFS disk usage in TAS for VMs

You can analyze GrootFS disk space usage in TAS for VMs. This article gives you important background and the steps to analyze GrootFS disk space usage.

You run the commands in this topic as root on any BOSH-deployed VM that hosts the Garden job in a VMware Tanzu Application Service for VMs (TAS for VMs) deployment.

This article provides different commands depending on whether you are using privileged or unprivileged containers in your deployment. By default, all deployments use unprivileged containers. For more information about these container types, see Container Security.

For more information about the GrootFS concepts, see GrootFS Disk Usage.

Reconcile Container Disk Usage and Host Disk Usage

To reconcile disk allocations for containers with the actual disk usage on the host VM, you need to understand how GrootFS uses its disk.

Command line tools such as du and df can provide misleading information because of the way container file systems work.

For example, the following situations can occur:

If...	Then...
the Diego cell rep appears to be out of disk capacity, but the actual disk usage on the Garden host is low	Diego does not schedule containers on the cell.
the Diego cell rep appears to have available disk capacity, but the combined space used by containers and system components prevents Diego from allocating the remaining disk space	Diego continues to place containers on the cell, but they fail to start.

About Container Disk Usage

On disk, the read-write layer for each container can be found at /var/vcap/data/grootfs/store/unprivileged/images/CONTAINER-ID/diff.

When GrootFS calls on the built-in XFS quota tooling to get disk usage for a container, it takes into account data written to that directory and not the data in the read-only volumes.

Running grootfs stats returns the following values:

total_bytes_used: This is the disk usage of the container including the rootfs image volumes.
exclusive_bytes_used: This is the disk usage of the container not including the rootfs image volumes.

Retrieve Disk Usage Stats for a Single Container

To obtain the disk usage stats of a single container, perform the following steps:

In your TAS for VMs deployment, use bosh ssh to connect to the BOSH-deployed VM running the Garden job.
On the VM, run the following command to look up the container ID:
```
ls /var/vcap/data/garden/depot/
```
The command above returns a container ID in the following format:
```
55afbf65-5cbf-49c6-4461-f803
```

Based on the container type used in your deployment, run one of the following commands on the VM:

For unprivileged containers, run the following command:

/var/vcap/packages/grootfs/bin/grootfs --config \
/var/vcap/jobs/garden/config/grootfs_config.yml stats CONTAINER-ID

Where CONTAINER-ID corresponds to the container ID you obtained in step 2.
For example:

$ /var/vcap/packages/grootfs/bin/grootfs --config \
/var/vcap/jobs/garden/config/grootfs_config.yml stats 55afbf65-5cbf-49c6-4461-f803

For privileged containers, run the following command:

/var/vcap/packages/grootfs/bin/grootfs --config \
/var/vcap/jobs/garden/config/privileged_grootfs_config.yml stats CONTAINER-ID

Where CONTAINER-ID corresponds to the container ID you obtained in step 2.

For example:

$ /var/vcap/packages/grootfs/bin/grootfs --config \
  /var/vcap/jobs/garden/config/privileged_grootfs_config.yml stats 55afbf65-5cbf-49c6-4461-f803

The commands above return output in the following format:

{"disk_usage":{"total_bytes_used":23448093,"exclusive_bytes_used":8192}}

Retrieve exclusive disk usage stats for all running containers

To check the disk usage of all running containers, perform the following steps:

In your TAS for VMs deployment, use bosh ssh to connect to the BOSH-deployed VM running the Garden job.

Based on the container type used in your deployment, run one of the following commands on the VM:

For unprivileged containers, run the following command:

ls /var/vcap/data/garden/depot/ \
| xargs -I{} /var/vcap/packages/grootfs/bin/grootfs --config \
/var/vcap/jobs/garden/config/grootfs_config.yml stats {} \
| cut -d: -f4 | cut -d} -f1 | awk '{sum += $1} END {print sum}'

For privileged containers, run the following command:

ls /var/vcap/data/garden/depot/ \
| xargs -I{} /var/vcap/packages/grootfs/bin/grootfs --config \
/var/vcap/jobs/garden/config/privileged_grootfs_config.yml stats {} \
| cut -d: -f4 | cut -d} -f1 | awk '{sum += $1} END {print sum}'

The commands above return the total disk usage in bytes for all running containers.

About Volumes in GrootFS

Underlying layers are known as volumes in GrootFS.

They are read-only and their changesets are layered together through an OverlayFS mount to create the rootfs for containers. When GrootFS writes each file system volume to disk, it also stores the number of bytes written to a file in the meta directory.

Check volume disk size

To find out the size of an individual volume, you can read the corresponding metadata file or run du on the volume itself. Perform the following steps:

In your TAS for VMs deployment, use bosh ssh to connect to the BOSH-deployed VM running the Garden job.
Based on the container type used in your deployment, run one of the following commands on the VM:
- For unprivileged containers, run the following command on the VM:
```
cat /var/vcap/data/grootfs/store/unprivileged/meta/volume-VOLUME-SHA
```
  Where VOLUME-SHA corresponds to the SHA value of the volume.
- For privileged containers, run the following command:
```
cat /var/vcap/data/grootfs/store/privileged/meta/volume-VOLUME-SHA
```
  Where VOLUME-SHA corresponds to the SHA value of the volume.
The cat commands above return the volume size in bytes in the following format:
```
{"Size":5607885}
```
Alternatively, use du and pass the absolute path to the volume. Run one of the following commands on the VM:
- For unprivileged containers, run the following command:
```
du -sch /var/vcap/data/grootfs/store/unprivileged/volumes/VOLUME-SHA/
```
  Where VOLUME-SHA corresponds to the SHA value of the volume.
- For privileged containers, run the following command:
```
du -sch /var/vcap/data/grootfs/store/privileged/volumes/VOLUME-SHA/
```
  Where VOLUME-SHA corresponds to the SHA value of the volume.
The du commands above return the volume size in the following format:
```
5.4M    /var/vcap/data/grootfs/store/unprivileged/volumes/VOLUME-SHA/
```

Determine disk usage and reclaimable disk space

This section describes how to calculate the amount of disk space is in use and estimate how much space is reclaimable.

Calculate disk use by all active volumes

For each container, GrootFS mounts the underlying volumes using overlay to a point in the images directory. This point is the rootfs for the container and is read-write.

GrootFS also stores the SHA of each underlying volume used by an image in the meta folder.

You can determine the bytes of all active volumes on disk by running one of the following commands:

If you do not have python3 installed, replace python3 with python in the following commands.

For unprivileged containers, run the following command:

for image in $(ls /var/vcap/data/grootfs/store/unprivileged/meta/dependencies/image\:*.json); \
do cat $image | python3 -c 'import json,sys;obj=json.load(sys.stdin); \
print("\n".join(obj))' ; done | sort -u \
| xargs -I{} cat /var/vcap/data/grootfs/store/unprivileged/meta/volume-{} \
| cut -d : -f 2 | cut -d} -f1 \
| awk '{sum += $1} END {print sum}'

For privileged containers, run the following command:

for image in $(ls /var/vcap/data/grootfs/store/privileged/meta/dependencies/image\:*.json); \
do cat $image | python3 -c 'import json,sys;obj=json.load(sys.stdin); \
print("\n".join(obj))' ; done | sort -u \
| xargs -I{} cat /var/vcap/data/grootfs/store/privileged/meta/volume-{} \
| cut -d : -f 2 | cut -d} -f1 \
| awk '{sum += $1} END {print sum}'

The commands above return the total number of bytes used by all active volumes on disk.

Calculate GrootFS store disk usage

To determine how much total disk space the store is using, run the following command:

df | grep -E  "/var/vcap/data/grootfs/store/(privileged|unprivileged)$" \
| awk '{sum += $3} END {print sum}'

The command above returns the total number of bytes used by the store.

Calculate reclaimable disk space

You can use values gathered from the commands above to calculate how much space can be cleared in GrootFS. Garbage collection reclaims disk space by pruning unused volumes.

The overall formula to calculate reclaimable disk space is to subtract the total disk in use by the store from the total disk used by active volumes.

For example, perform the following steps:

Calculate how much disk space the store is using by following the instructions in Calculate GrootFS Store Disk Usage. For example, your result might be:
```
Total disk store = 5607885 bytes
```
Calculate how much disk space active volumes are using by following the instructions in Calculate Disk Use by All Active Volumes. For example, your result might be:
```
Active volumes = 3212435 bytes
```
Subtract the amount of space used by active volumes from the space used by the store. For example, your result might be:
```
5607885 - 3212435 = 2395450 bytes
```

In this example, you can reclaim 2395450 bytes through garbage collection.

How GrootFS reclaims disk space

The thresholder component calculates and sets a value so that GrootFS’s garbage collector can attempt to ensure that a small reserved space is kept free for other jobs. GrootFS only tries to garbage collect or reclaim space when that threshold is reached. However, if all the rootfs layers are actively in use by images, then garbage collection cannot occur and that space is used up.

If you determine that there is not enough reclaimable disk and more space is needed on disk, you should scale up your VMs to a larger size or add more VMs to provide more disk space.

Alternatively, you can configure a lower threshold for Diego Cell disk cleanup in TAS for VMs. For more information, see Configuring Diego Cell Disk Cleanup Scheduling.

Other categories of GrootFS disk usage

There may be categories of GrootFS disk usage other than those listed in the above sections. However, the bulk of disk usage is stored in the images/CONTAINER-ID/diff and volumes directories, so these are rarely taken into consideration when calculating store usage.

You can find these directories under /var/vcap/data/grootfs/store/unprivileged for unprivileged containers and /var/vcap/data/grootfs/store/privileged for privileged containers.

GrootFS also stores information in the following directories:

l: link directories. Shorter directory names are symlinked to volume directories to allow Groot to union mount more file paths.
locks: file system lock directory to ensure safety during concurrent cleans and creates.
meta: per image and volume metadata.
projectids: empty numbered directories used to track image quotas.
tmp: normal temporary directory contents.

These directories typically use less than 2 MB disk in total.