DOC add ES free disk space req.

server/sonar-docs/src/pages/requirements/hardware-recommendations.md

@@ -20,6 +20,7 @@ In case your SonarQube Server is running on Linux and you are using Oracle, the
 * It is recommended to set the min and max memory to the same value to prevent the heap from resizing at runtime, a very costly process. See -Xms and  -Xmx of property `sonar.search.javaOpts`.
 
 ## Disk
+* Free disk space is an absolute requirement. ES will lock indices in read-only mode if free disk space drops below 20%. 
 * Disk can easily become the bottleneck of ES. If you can afford SSDs, they are by far superior to any spinning media. SSD-backed nodes see boosts in both query and indexing performance. If you use spinning media, try to obtain the fastest disks possible (high performance server disks 15k RPM drives).
 * Make sure to increase the number of open files descriptors on the machine (or for the user running SonarQube server). Setting it to 32k or even 64k is recommended. See [this ElasticSearch article](https://www.elastic.co/guide/en/elasticsearch/reference/current/file-descriptors.html).
 * Using RAID 0 is an effective way to increase disk speed, for both spinning disks and SSD. There is no need to use mirroring or parity variants of RAID because of Elasticsearch replicas and database primary storage.