Elasticsearch ignored disk watermark settings and enforced read only index

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When Elasticsearch experiences a disk full event, its "defense mechanism" is to try to move shards to another cluster member or, when nothing helps, set its indexes to read-only.
By default Elasticsearch starts to react at 85% disk usage (filesystem usage). To change the thresholds, the parameters "cluster.routing.allocation.disk.watermark" can be used. I already wrote about this in an older article "ElasticSearch stopped to assign shards due to low disk space" back in December 2017. Back then I was using Elasticsearch 5.x.

This morning I came across a similar issue with Elasticsearch 6.5.4. In Kibana no new log entries were shown; the last entries were from Friday evening:

Elasticsearch stopped logging, seen in Kibana Graph

When I looked at the Elasticsearch log files, I found a lot of lines like this one:

[2019-01-28T08:52:55,038][ERROR][o.e.x.w.e.ExecutionService] [inf-elkesi01-p] could not store triggered watch with id [prOmO-8rSreTrgLGraNv6w_kibana_version_mismatch_672f0d26-e20b-40d0-a54c-006543689e7c-2019-01-28T07:52:55.036Z]: [ClusterBlockException[blocked by: [FORBIDDEN/12/index read-only / allow delete (api)];]]

So Elasticsearch decided to set the read-only flag. But why? Although disk usage was at 96%, I remembered that we set different watermark thresholds based on units, not percentage:

# df -h | grep elastic
/dev/mapper/vges-lves      ext4         3.9T  3.7T  168G  96% /var/lib/elasticsearch

# grep watermark /etc/elasticsearch/elasticsearch.yml
#cluster.routing.allocation.disk.watermark.low: "95%"
# 20180719: Set new watermarks
cluster.routing.allocation.disk.watermark.low: "100G"
cluster.routing.allocation.disk.watermark.high: "50G"
cluster.routing.allocation.disk.watermark.flood_stage: "10G"

Elasticsearch should only start to be aware of high disk usage when there's only 100GB left, yet it seems that the defaults were used. According to the documentation:

    Controls the low watermark for disk usage. It defaults to 85%

    Controls the high watermark. It defaults to 90%

    Controls the flood stage watermark. It defaults to 95%, meaning that Elasticsearch enforces a read-only index block on every index

Even though the watermark thresholds were defined in elasticsearch.yml, the cluster settings didn't show it:

# curl -s http://localhost:9200/_cluster/settings -u elastic:XXX

Note: I'm actually not sure if the parameters defined in the config file are supposed to show up in the API. Maybe this is normal. Couldn't find any info for this.

So I decided to tell the Elasticsearch cluster using the API:

# curl -X PUT -H "Content-Type: application/json" http://localhost:9200/_cluster/settings -u elastic:XXX -d '{ "persistent": { "cluster.routing.allocation.disk.watermark.low": "100gb", "cluster.routing.allocation.disk.watermark.high": "50gb", "cluster.routing.allocation.disk.watermark.flood_stage": "10gb", "cluster.info.update.interval": "1m" } }'

Note: I used a "persistent" setting to survive cluster restarts.

The watermark thresholds now show up in the API:

# curl -s http://localhost:9200/_cluster/settings -u elastic:XXX

But the cluster is still read only at this time. I decided to manually delete some older indexes and freed quite some space:

# df -h | grep elastic
/dev/mapper/vges-lves      ext4         3.9T  3.6T  279G  93% /var/lib/elasticsearch

So we're far away from the thresholds now yet still no new data.

I tried a full restart of the cluster and waited until the cluster was green again. But still, no data coming in. Logstash reports:

[2019-01-28T11:00:56,068][INFO ][logstash.outputs.elasticsearch] retrying failed action with response code: 403 ({"type"=>"cluster_block_exception", "reason"=>"blocked by: [FORBIDDEN/12/index read-only / allow delete (api)];"})

On my research I came across a blog post and this shed some lights into the read-only situation:

" [...] elasticsearch is switching to read-only if it cannot index more documents because your hard drive is full [...] Elasticsearch will not switch back automatically [...] "

Oh? So I basically need to tell Elasticsearch "hey, you're good and stop being read-only"? Let's do this:

# curl -XPUT -H "Content-Type: application/json" http://localhost:9200/_all/_settings -d '{"index.blocks.read_only_allow_delete": null}' -u elastic:XXX

Right after this, data started to arrive in ES again!

To sum it up, there are two issues and solutions here:

1) The watermark thresholds from the Elasticsearch config file were obviously ignored. As we have a support contract with ES, I will ask in a ticket why this happened. To work around this for now, I defined the thresholds manually in the Elasticsearch API using the persistent settings.

2) When Elasticsearch switches to read-only mode, it will not recover automatically. You will have to manually reset this setting in the API.

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