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This can include multiple issues, but the most common are that of exhaustion of free space on a device, or that of SD card corruption.

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11.1 Out of Space Issues

A media partition that is full can cause issues such as the following:
  • Failure to download release updates, or failure to start new/updated services after a download has occurred
  • Failure for a service to store data into defined volumes
  • Failure of services to start up (mostly those that need to store data that isn’t in tmpfs)
Determining how much space is left on the media for a device can be achieved by logging into the host OS and running: 
root@debug-device:~# df -h
Filesystem                      Size  Used Avail Use% Mounted on
devtmpfs                        783M     0  783M   0% /dev
tmpfs                           950M  5.3M  945M   1% /run
/dev/mmcblk0p2                  300M  276M  4.5M  99% /mnt/sysroot/active
/dev/disk/by-state/resin-state   18M   75K   16M   1% /mnt/state
overlay                         300M  276M  4.5M  99% /
/dev/mmcblk0p6                   29G  367M   27G   2% /mnt/data
tmpfs                           950M     0  950M   0% /dev/shm
tmpfs                           4.0M     0  4.0M   0% /sys/fs/cgroup
tmpfs                           950M     0  950M   0% /tmp
tmpfs                           950M   40K  950M   1% /var/volatile
/dev/mmcblk0p1                   40M  7.2M   33M  19% /mnt/boot
/dev/mmcblk0p3                  300M   14K  280M   1% /mnt/sysroot/inactive
The -h switch makes the figures returned ‘human readable’. Without this switch the returned figures will be in block sizes (usually 1k or 512byte blocks). The two main mounts where full space problems commonly occur are /mnt/data and /mnt/state. The former is the data partition where all service images, containers and volumes are stored. The latter is the state partition, where overlays for the root FS (such as user defined network configurations) and the permanent logs are stored. There are a few ways to try and relieve out of space issues on a media drive.
11.1.1 Image and Container Pruning
One fairly easy cleanup routine to perform is that of pruning the Docker tree so that any unused images, containers, networks and volumes are removed. It should be noted that in the day-to-day operation of the Supervisor, it attempts to ensure that anything that is no longer used on the device is removed when not required. However, there are issues that sometimes occur that can cause this behavior to not work correctly. In these cases, a prune should help clean anything that should not be present: 
root@debug-device:~# balena system prune -a -f --volumes
Deleted Images:
untagged: balena-healthcheck-image:latest
deleted: sha256:46331d942d6350436f64e614d75725f6de3bb5c63e266e236e04389820a234c4
deleted: sha256:efb53921da3394806160641b72a2cbd34ca1a9a8345ac670a85a04ad3d0e3507
untagged: balena_supervisor:v14.0.8

Total reclaimed space: 9.136kB
Note that in the above, all unused images, containers, networks and volumes will be removed. To just remove dangling images, you can use balena system prune -a.
11.1.2 Customer Data
Occasionally, customer volumes can also fill up the data partition. This obviously causes more issues, because usually this is data that cannot just be deleted. In these cases, it’s imperative that the customer is informed that they’ve filled the data partition and that appropriate pruning is required. Filling disk space does not tend to stop access to devices, so in these cases customers should be asked to enter the relevant services and manually prune data. Before discussion on persistent data, it’s worth noting that occasionally customer apps store data to the service container instead of a persistent data volume. Sometimes, this data is intended as temporary, so doing so is not an issue (although if they are doing so and expecting it to stay permanent, this will not occur as service container rebuilds will remove the layers where new data is stored). However there are cases where even this temporary data can be so large that it fills the storage media. In these cases, the Supervisor can be stopped, and then the service container affected, allowing that container to be removed so the Supervisor can rebuild from the service image. This will remove the layers filling the space. Care should be taken and customers informed first, in case this data is required. They should also be informed of persistent data and how to use it. Because persistent data is stored as volumes, it’s also possible to prune data for a service from within the host OS. For example, should a service be filling a volume so quickly as to prevent sensible data removal, an option is to stop that service and then manually remove data from the service’s volume. Data volumes are always located in the /var/lib/docker/volumes directory. Care needs to be taken to ensure the right volumes are examine/pruned of data, as not all volumes pertain directly to customer data. Let’s list the volumes: 
root@debug-device:~# ls -l /var/lib/docker/volumes/
total 28
drwx-----x 3 root root  4096 Aug 19 19:15 1958513_backend-data
-rw------- 1 root root 32768 Aug 19 19:15 metadata.db
In single service apps, the relevant data volume is suffixed with the _balena-data string. In multicontainer apps, the suffix always corresponds with the name of the bound volume. For example, let’s look at the docker-compose manifest for the multicontainer-app app used in this debugging masterclass: 
version: '2.1'
volumes:
  backend-data: {}
services:
  frontend:
    build: ./frontend
    network_mode: host
  backend:
    build: ./backend
    labels:
      io.balena.features.supervisor-api: '1'
      io.balena.features.balena-api: '1'
    privileged: true
    volumes:
      - 'backend-data:/mydata'
As you can see, a backend-data volume is defined, and then used by the backend service. Assuming your device is still running the multicontainer app for this masterclass, SSH into the device, and then examine the running services: 
root@debug-device:~# balena ps
CONTAINER ID   IMAGE                                                            COMMAND                  CREATED              STATUS                    PORTS     NAMES
330d34540489   3128dae78199                                                     "/usr/bin/entry.sh n…"   About a minute ago   Up About a minute                   backend_5302053_2266082_28d1b0e8e99c2ae6b7361f3b0f835f5c
2e2a7fcfe6f6   f0735c857f39                                                     "/usr/bin/entry.sh n…"   57 minutes ago       Up 16 minutes                       frontend_5302052_2266082_28d1b0e8e99c2ae6b7361f3b0f835f5c
e593ab6439fe   registry2.balena-cloud.com/v2/04a158f884a537fc1bd11f2af797676a   "/usr/src/app/entry.…"   57 minutes ago       Up 16 minutes (healthy)             balena_supervisor
root@debug-device:~# balena inspect backend_5302053_2266082_28d1b0e8e99c2ae6b7361f3b0f835f5c | grep /var/lib/docker/volumes
                "Source": "/var/lib/docker/volumes/1958513_backend-data/_data",
The volume is denoted with the suffix of the defined volume name. Should there be multiple volumes, then appropriate directories for these will be created in the /var/lib/docker/volumes directory, with the relevant suffixes. Knowing this, it becomes fairly simple to stop services that have filled volumes and to clear these out:
1
Stop the Supervisor and start timer (balena-supervisor.service and update-balena-supervisor.timer).
2
Determine the relevant data directories for the volumes filling the data partition.
3
Clean them appropriately.
4
Restart the Supervisor and start timer.

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11.2 Storage Media Corruption

Many device types use storage media that has high wear levels. This includes devices such as the Raspberry Pi series, where SD cards are the usual storage media. Because we recommend very hard-wearing cards (the SanDisk Extreme Pro family are extremely resilient), we don’t regularly have issues with customer devices dying due to SD card failure. However, they do occur (and not just on SD cards, any type of flash memory based storage includes a shorter lifespan compared to media such as platter drives). Initially, media corruption and wearing exhibit ‘random’ signs, including but not limited to:
  • Release updates failing to download/start/stop.
  • Services suddenly restarting.
  • Devices not being mapped to device nodes.
  • Extreme lag when interacting with services/utilities from the CLI.
  • Spurious kernel errors.
In fact, media corruption could potentially exhibit as any sort of issue, because there’s (generally) no way to determine where wearing may exhibit itself. Additionally, we have seen issues where media write/reads take so long that they also adversely impact the system (for example, healthchecks may take too long to occur, which could potentially restart services including the Supervisor and balenaEngine), in these cases swapping the media for a known, working brand has caused this issues to be resolved. One quick check that can be carried out is the root filing system integrity check. This checks the MD5 hashes fingerprints of all the files in the filing system against those when they were built. This tends to give an idea of whether corruption may be an issue (but it certainly isn’t guaranteed). SSH into your device and run the following: 
root@debug-device:~# grep -v "/var/cache/ldconfig/aux-cache" /balenaos.fingerprint | md5sum --quiet -c -
If the check returns successfully, none of the files differ in their MD5 fingerprints from when they were built. Generally, if it appears that media corruption may be an issue, we generally check with customers if they’re running a recommended media brand, and if not ask them to do so. Should the worst happen and a device is no longer bootable due to filesystem corruption, they still have the option of recovering data from the device. In this case, they’ll need to remove the media (SD card, HDD, etc.) from the device and then follow appropriate instructions. Similar commands to check for storage corruption although can sometime provide false positives. 
journalctl -n 10000 | grep "corrupt|Data will be lost|block allocation failed|invalid magic|dockerd: error -117|EXT4-fs error|error count since last fsck|failed checksum|checksum invalid|Please run e2fsck|I/O error"
Messages from the command output should be carefully observed and verified to troubleshoot storage corruption.