pikvm/docs/pst.md

Persistent storage

??? note This feature is available on images older than 2022.06.20

Sometimes advanced use of PiKVM requires storing some data on disk like API keys, config files, or something like that. For example, you want to have a script that will update SSL certificates once a week. However, the root file system is in a read-only state and does not involve remounting automatically by user scripts.

To solve this problem, new versions of PiKVM have a small 256MiB storage partition that can be used to store that data. A special kvmd-pst daemon makes sure that this partition is mounted in read-only all the time, and remounts it to RW only when some user script requires it. This also solves the problems of simultaneous access, so the RW mode will be keeped as long as at least one client is working with the storage.

Usage

Below is an example of a script /root/test.sh that wants to save a certain file in PST:

#!/bin/bash
echo `date` + $@ > $KVMD_PST_DATA/foo
cat $KVMD_PST_DATA/foo

To run it use:

# kvmd-pstrun -- /root/test.sh --some --script --args
--    INFO -- Opening PST session ...
--    INFO -- PST write is allowed: /var/lib/kvmd/pst/data
--    INFO -- Running the process ...
Mon Jun 20 04:23:14 MSK 2022 + --some --script --args
--    INFO -- Process finished: returncode=0

So, what's going on here:

1. kvmd-pstrun connects to the kvmd-pst daemon, which manages the mounting of the storage.
2. If everything is fine, the daemon will remount the storage to RW mode and report the data root to kvmd-pstrun.
3. kvmd-pstrun runs the script and pass the data root path using the environment variable KVMD_PST_DATA (/var/lib/kvmd/pst/data).
4. If the kvmd-pst daemon stops or any other daemon error occurs, the script will be killed.
5. After the script is finished, the daemon will remount the storage to RO mode.