SonarQube would not start

Context:
- Windows 10
- SonarQube 7.7
- Java:
java version "1.8.0_211"
Java(TM) SE Runtime Environment (build 1.8.0_211-b12)
Java HotSpot(TM) 64-Bit Server VM (build 25.211-b12, mixed mode)

Have been installing SonarQube. Faced an issue where SonarQube wouldn't start. I.e. StartSonar.bat would exit without reporting a meaningful error. It was just listing several processes that were terminating without giving a reason. The first process to terminate was "web". Found the web's process' log here:

[SONARQUBE HOME DIR]\logs\web.log

It contained a more useful error message:

2019.04.18 18:22:49 ERROR web[][o.s.s.p.Platform] Background initialization failed. Stopping SonarQube
org.elasticsearch.cluster.block.ClusterBlockException: blocked by: [FORBIDDEN/12/index read-only / allow delete (api)];
at org.elasticsearch.cluster.block.ClusterBlocks.indexBlockedException(ClusterBlocks.java:183)
at org.elasticsearch.action.support.replication.TransportReplicationAction.blockExceptions(TransportReplicationAction.java:255)
at org.elasticsearch.action.support.replication.TransportReplicationAction.access$500(TransportReplicationAction.java:100)
at org.elasticsearch.action.support.replication.TransportReplicationAction$ReroutePhase.doRun(TransportReplicationAction.java:780)
at org.elasticsearch.common.util.concurrent.AbstractRunnable.run(AbstractRunnable.java:37)
at org.elasticsearch.action.support.replication.TransportReplicationAction.doExecute(TransportReplicationAction.java:172)
at org.elasticsearch.action.support.replication.TransportReplicationAction.doExecute(TransportReplicationAction.java:100)
at org.elasticsearch.action.support.TransportAction$RequestFilterChain.proceed(TransportAction.java:167)
at org.elasticsearch.action.support.TransportAction.execute(TransportAction.java:139)
at org.elasticsearch.action.support.TransportAction.execute(TransportAction.java:81)
at org.elasticsearch.action.bulk.TransportBulkAction$BulkOperation.doRun(TransportBulkAction.java:420)
at org.elasticsearch.common.util.concurrent.AbstractRunnable.run(AbstractRunnable.java:37)
at org.elasticsearch.action.bulk.TransportBulkAction.executeBulk(TransportBulkAction.java:533)
at org.elasticsearch.action.bulk.TransportBulkAction.executeIngestAndBulk(TransportBulkAction.java:271)
at org.elasticsearch.action.bulk.TransportBulkAction.doExecute(TransportBulkAction.java:222)
at org.elasticsearch.action.bulk.TransportBulkAction.doExecute(TransportBulkAction.java:90)
at org.elasticsearch.action.support.TransportAction$RequestFilterChain.proceed(TransportAction.java:167)
at org.elasticsearch.action.support.TransportAction.execute(TransportAction.java:139)
at org.elasticsearch.action.bulk.TransportSingleItemBulkWriteAction.doExecute(TransportSingleItemBulkWriteAction.java:69)
at org.elasticsearch.action.bulk.TransportSingleItemBulkWriteAction.doExecute(TransportSingleItemBulkWriteAction.java:44)
at org.elasticsearch.action.support.TransportAction$RequestFilterChain.proceed(TransportAction.java:167)
at org.elasticsearch.action.support.TransportAction.execute(TransportAction.java:139)
at org.elasticsearch.action.support.replication.TransportReplicationAction$OperationTransportHandler.messageReceived(TransportReplicationAction.java:284)
at org.elasticsearch.action.support.replication.TransportReplicationAction$OperationTransportHandler.messageReceived(TransportReplicationAction.java:276)
at org.elasticsearch.transport.RequestHandlerRegistry.processMessageReceived(RequestHandlerRegistry.java:66)
at org.elasticsearch.transport.TcpTransport$RequestHandler.doRun(TcpTransport.java:1289)
at org.elasticsearch.common.util.concurrent.AbstractRunnable.run(AbstractRunnable.java:37)
at org.elasticsearch.common.util.concurrent.EsExecutors$1.execute(EsExecutors.java:140)
at org.elasticsearch.transport.TcpTransport.handleRequest(TcpTransport.java:1247)
at org.elasticsearch.transport.TcpTransport.messageReceived(TcpTransport.java:1111)
at org.elasticsearch.transport.TcpTransport.inboundMessage(TcpTransport.java:914)
at org.elasticsearch.transport.netty4.Netty4MessageChannelHandler.channelRead(Netty4MessageChannelHandler.java:53)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:340)
at io.netty.handler.codec.ByteToMessageDecoder.fireChannelRead(ByteToMessageDecoder.java:323)
at io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:297)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:340)
at io.netty.handler.logging.LoggingHandler.channelRead(LoggingHandler.java:241)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:340)
at io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1434)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:965)
at io.netty.channel.nio.AbstractNioByteChannel$NioByteUnsafe.read(AbstractNioByteChannel.java:163)
at io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:656)
at io.netty.channel.nio.NioEventLoop.processSelectedKeysPlain(NioEventLoop.java:556)
at io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:510)
at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:470)
at io.netty.util.concurrent.SingleThreadEventExecutor$5.run(SingleThreadEventExecutor.java:909)
at java.lang.Thread.run(Unknown Source)

Googling revealed that the most likely cause of it is lack of disk space for the Elasticseach engine used by SonarQube. However, simply cleaning up more space (I increased available space from 7GB to 80GB) was not enough. I had to also remove this folder:

[SONARQUBE HOME DIR>]\data\es6

After removing this folder and restarting SonarQube, it started fine. The es6 directory got re-created.

The reason for this behavior seems to be that, when Elasticsearch detects there is not enough disk space, it marks 'indexes' as R/O and this somehow gets remembered in the es6 directory. So even after cleaning up space, the 'indexes' are still R/O.