Parity Spot Check for Unraid

Continuously spot-checks your array's parity in the background: it picks a random location on the array, runs Unraid's own parity-check engine over just that stretch, pauses, picks a new random location, and repeats — indefinitely, until you turn it off.

Copyright © 2026 Ray Munro. Licensed under the GNU GPLv3.

How it works

Unraid's mdcmd check supports an optional starting sector offset (mdcmd check NOCORRECT <offset>). Parity Spot Check uses exactly that official interface — it does not reimplement parity math or read disks directly. Each cycle it:

1. Confirms the array is started, the parity disk is reporting DISK_OK, and no other check/rebuild is already running.
2. Picks a random sector offset within the parity-covered range.
3. Starts a read-only (NOCORRECT) check at that offset.
4. Polls /var/local/emhttp/var.ini until roughly the configured "spot size" of data has been verified, then pauses the check (mdcmd nocheck).
5. Records the result (mismatches found, if any) and rests for the configured interval before picking a new random offset.

Safety

• Always NOCORRECT. This plugin never writes to parity and has no setting to enable correcting mode. It only ever reports mismatches.
• Never interferes with a real check. If a parity check or disk rebuild is already running (started by you or by Unraid itself), the daemon waits — it never cancels or takes over someone else's operation.
• Stopping is immediate. Disabling the plugin or stopping its service always issues mdcmd nocheck, so an in-flight spot check is paused right away, not left running in the background.
• If a mismatch is found, Unraid sends a Warning notification through your configured notification agents. Investigate before running a normal correcting parity check yourself from the Main page.

Limitations

• The random-offset behaviour depends on mdcmd check honouring the optional offset argument. This is confirmed present in current Unraid releases; if your version predates it, spot checks will effectively always start from sector 0. Check the system log (logger -t parity-spotcheck, visible in Tools → System Log) to confirm offsets are actually varying.
• A parity check reads all data/parity disks together for each stripe, so spot checks exercise the whole array at that offset, not one disk.
• Live array writes during a spot check can, in rare cases, look like a false-positive mismatch — the same caveat that applies to any Unraid parity check run while the array is in use.
• Counters and history on the settings page are kept in memory/tmpfs and reset on reboot or plugin restart; the system log is the durable trail.

Installation

Via Community Applications: search for "Parity Spot Check" in the Apps tab and click Install.

Manually: in the Unraid webGUI go to Plugins → Install Plugin and paste:

https://raw.githubusercontent.com/RayMunro/unraid-parity-spotcheck/main/parity-spotcheck.plg

or from the terminal:

plugin install https://raw.githubusercontent.com/RayMunro/unraid-parity-spotcheck/main/parity-spotcheck.plg

Then open Settings → User Utilities → Parity Spot Check, set a spot size and rest interval, and enable it. Everything the plugin needs is written by the .plg itself, so nothing else needs to be downloaded or copied by hand.

Configuration

• Spot size (MB) — how much of the array each spot check covers (default 1024 MB / 1 GB).
• Rest between spot checks (seconds) — idle time between spots so disks aren't hammered continuously (default 300s).

Uninstall

From Settings → Plugins, remove "parity-spotcheck" normally, or run:

plugin remove parity-spotcheck.plg

Any in-flight spot check is paused automatically as part of removal.