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51 lines
2.0 KiB
Markdown
51 lines
2.0 KiB
Markdown
# Persistent storage
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!!! note
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This feature is available on images newer than 2022.06.20
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Sometimes advanced use of PiKVM requires storing some data on disk like API keys, config files, or something like that.
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For example, you want to have a script that will update SSL certificates once a week.
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However, the root file system is in a read-only state and does not involve remounting automatically by user scripts.
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To solve this problem, new versions of PiKVM have a small 256MiB storage partition that can be used to store that data.
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A special `kvmd-pst` daemon makes sure that this partition is mounted in read-only all the time, and remounts it to RW
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only when some user script requires it. This also solves the problems of simultaneous access, so the RW mode will be
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kept as long as at least one client is working with the storage.
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## Usage
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Below is an example of a script `/root/test.sh` that wants to save a certain file in PST:
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```bash
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#!/bin/bash
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echo `date` + $@ > $KVMD_PST_DATA/foo
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cat $KVMD_PST_DATA/foo
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```
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To run it use:
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```
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# kvmd-pstrun -- /root/test.sh --some --script --args
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-- INFO -- Opening PST session ...
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-- INFO -- PST write is allowed: /var/lib/kvmd/pst/data
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-- INFO -- Running the process ...
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Mon Jun 20 04:23:14 MSK 2022 + --some --script --args
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-- INFO -- Process finished: returncode=0
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```
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So, what's going on here:
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1. `kvmd-pstrun` connects to the `kvmd-pst` daemon, which manages the mounting of the storage.
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2. If everything is fine, the daemon will remount the storage to RW mode and report the data root to `kvmd-pstrun`.
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3. `kvmd-pstrun` runs the script and pass the data root path using the environment variable `KVMD_PST_DATA` (`/var/lib/kvmd/pst/data`).
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4. If the `kvmd-pst` daemon stops or any other daemon error occurs, the script will be killed.
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5. After the script is finished, the daemon will remount the storage to RO mode.
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To be able to write to the storage, the user must be a member of `kvmd-pst` group, or have to be `root`.
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The return code will be equal to the script code if it was run, or 1 if a remount error occurred.
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