## TL;DR
OpenSearch flips indices to read-only when disk hits the flood-stage watermark, to protect the cluster. Free disk space (delete old indices or add capacity), then clear the `index.blocks.read_only_allow_delete` setting. Writes resume once the block is lifted.

## Error
```text
opensearch: cluster_block_exception index read-only after disk watermark
```

## Steps
1. Confirm disk pressure: check disk usage on the data nodes. Expected: usage at or above the flood-stage watermark.
2. Free space: delete old indices, force-merge, or add disk capacity. Expected: usage drops below the watermark with margin.
3. Clear the block: update the index settings to set `index.blocks.read_only_allow_delete` to false (all indices, or the affected ones). Expected: the setting clears.
4. Verify writes: index a test document. Expected: writes succeed without the block exception.
5. Prevent recurrence: set up index lifecycle management and disk alerting well below the flood stage. Expected: the watermark never triggers unnoticed again.

## When to use
- Writes fail with `cluster_block_exception` and read-only indices
- After disk filled on data nodes
- Setting up retention to prevent recurrence

## When not to use
- Unassigned shards (allocation issue)
- The block was set manually (just clear the setting)

## Tool compatibility
- OpenSearch index settings (`index.blocks.read_only_allow_delete`); disk watermarks, ILM

## Variant phrasings
### opensearch disk watermark read only
Free disk first; clearing the block without headroom just re-triggers it.

### flood stage watermark opensearch
Delete old data or add capacity, then lift the block.

## Why it happens
The flood-stage watermark is a safety brake: rather than letting disks fill completely (which corrupts the cluster), OpenSearch makes indices read-only. The block persists even after disk frees up, so it must be cleared explicitly.

## Provenance

Resolved from the public thread: https://vectle.com/posts/pst_iIRFPkyx87oXzFK9O1MtAw