Threadpool merge executor is aware of available disk space (#127613)

This PR introduces 3 new settings:
indices.merge.disk.check_interval, indices.merge.disk.watermark.high, and indices.merge.disk.watermark.high.max_headroom
that control if the threadpool merge executor starts executing new merges when the disk space is getting low.

The intent of this change is to avoid the situation where in-progress merges exhaust the available disk space on the node's local filesystem.
To this end, the thread pool merge executor periodically monitors the available disk space, as well as the current disk space estimates required by all in-progress (currently running) merges on the node, and will NOT schedule any new merges if the disk space is getting low (by default below the 5% limit of the total disk space, or 100 GB, whichever is smaller (same as the disk allocation flood stage level)).

Author: albertzaharovits

docs/changelog/127613.yaml

@@ -0,0 +1,5 @@
+pr: 127613
+summary: Threadpool merge executor is aware of available disk space
+area: Engine
+type: feature
+issues: []

server/src/main/java/org/elasticsearch/common/settings/ClusterSettings.java

@@ -87,6 +87,7 @@
 import org.elasticsearch.index.IndexSettings;
 import org.elasticsearch.index.IndexingPressure;
 import org.elasticsearch.index.MergePolicyConfig;
+import org.elasticsearch.index.engine.ThreadPoolMergeExecutorService;
 import org.elasticsearch.index.engine.ThreadPoolMergeScheduler;
 import org.elasticsearch.indices.IndexingMemoryController;
 import org.elasticsearch.indices.IndicesQueryCache;
@@ -621,6 +622,9 @@ public void apply(Settings value, Settings current, Settings previous) {
         MergePolicyConfig.DEFAULT_MAX_MERGED_SEGMENT_SETTING,
         MergePolicyConfig.DEFAULT_MAX_TIME_BASED_MERGED_SEGMENT_SETTING,
         ThreadPoolMergeScheduler.USE_THREAD_POOL_MERGE_SCHEDULER_SETTING,
+        ThreadPoolMergeExecutorService.INDICES_MERGE_DISK_HIGH_WATERMARK_SETTING,
+        ThreadPoolMergeExecutorService.INDICES_MERGE_DISK_HIGH_MAX_HEADROOM_SETTING,
+        ThreadPoolMergeExecutorService.INDICES_MERGE_DISK_CHECK_INTERVAL_SETTING,
         TransportService.ENABLE_STACK_OVERFLOW_AVOIDANCE,
         DataStreamGlobalRetentionSettings.DATA_STREAMS_DEFAULT_RETENTION_SETTING,
         DataStreamGlobalRetentionSettings.DATA_STREAMS_MAX_RETENTION_SETTING,

server/src/main/java/org/elasticsearch/index/engine/ThreadPoolMergeExecutorService.java

@@ -55,7 +55,7 @@ public class ThreadPoolMergeScheduler extends MergeScheduler implements Elastics
     private final ThreadPoolMergeExecutorService threadPoolMergeExecutorService;
     private final PriorityQueue<MergeTask> backloggedMergeTasks = new PriorityQueue<>(
         16,
-        Comparator.comparingLong(MergeTask::estimatedMergeSize)
+        Comparator.comparingLong(MergeTask::estimatedRemainingMergeSize)
     );
     private final Map<MergePolicy.OneMerge, MergeTask> runningMergeTasks = new HashMap<>();
     // set when incoming merges should be throttled (i.e. restrict the indexing rate)
@@ -214,7 +214,7 @@ private void checkMergeTaskThrottling() {
     // exposed for tests
     // synchronized so that {@code #closed}, {@code #runningMergeTasks} and {@code #backloggedMergeTasks} are modified atomically
     synchronized Schedule schedule(MergeTask mergeTask) {
-        assert mergeTask.isRunning() == false;
+        assert mergeTask.hasStartedRunning() == false;
         if (closed) {
             // do not run or backlog tasks when closing the merge scheduler, instead abort them
             return Schedule.ABORT;
@@ -223,6 +223,7 @@ synchronized Schedule schedule(MergeTask mergeTask) {
             assert added : "starting merge task [" + mergeTask + "] registered as already running";
             return Schedule.RUN;
         } else {
+            assert mergeTask.hasStartedRunning() == false;
             backloggedMergeTasks.add(mergeTask);
             return Schedule.BACKLOG;
         }
@@ -337,8 +338,14 @@ public void setIORateLimit(long ioRateLimitBytesPerSec) {
             this.rateLimiter.setMBPerSec(ByteSizeValue.ofBytes(ioRateLimitBytesPerSec).getMbFrac());
         }
 
-        public boolean isRunning() {
-            return mergeStartTimeNS.get() > 0L;
+        /**
+         * Returns {@code true} if this task is currently running, or was run in the past.
+         * An aborted task (see {@link #abort()}) is considered as NOT run.
+         */
+        public boolean hasStartedRunning() {
+            boolean isRunning = mergeStartTimeNS.get() > 0L;
+            assert isRunning != false || rateLimiter.getTotalBytesWritten() == 0L;
+            return isRunning;
         }
 
         /**
@@ -349,7 +356,7 @@ public boolean isRunning() {
          */
         @Override
         public void run() {
-            assert isRunning() == false;
+            assert hasStartedRunning() == false;
             assert ThreadPoolMergeScheduler.this.runningMergeTasks.containsKey(onGoingMerge.getMerge())
                 : "runNowOrBacklog must be invoked before actually running the merge task";
             try {
@@ -414,7 +421,7 @@ public void run() {
          * (by the {@link org.apache.lucene.index.IndexWriter}) to any subsequent merges.
          */
         void abort() {
-            assert isRunning() == false;
+            assert hasStartedRunning() == false;
             assert ThreadPoolMergeScheduler.this.runningMergeTasks.containsKey(onGoingMerge.getMerge()) == false
                 : "cannot abort a merge task that's already running";
             if (verbose()) {
@@ -443,10 +450,17 @@ void abort() {
             }
         }
 
-        long estimatedMergeSize() {
+        /**
+         * Before the merge task started running, this returns the estimated required disk space for the merge to complete
+         * (i.e. the estimated disk space size of the resulting segment following the merge).
+         * While the merge is running, the returned estimation is updated to take into account the data that's already been written.
+         * After the merge completes, the estimation returned here should ideally be close to "0".
+         */
+        long estimatedRemainingMergeSize() {
             // TODO is it possible that `estimatedMergeBytes` be `0` for correctly initialize merges,
             // or is it always the case that if `estimatedMergeBytes` is `0` that means that the merge has not yet been initialized?
-            return onGoingMerge.getMerge().getStoreMergeInfo().estimatedMergeBytes();
+            long estimatedMergeSize = onGoingMerge.getMerge().getStoreMergeInfo().estimatedMergeBytes();
+            return Math.max(0L, estimatedMergeSize - rateLimiter.getTotalBytesWritten());
         }
 
         @Override

server/src/main/java/org/elasticsearch/index/engine/ThreadPoolMergeScheduler.java

@@ -289,10 +289,6 @@ protected void doStart() {
     IndicesService(IndicesServiceBuilder builder) {
         this.settings = builder.settings;
         this.threadPool = builder.threadPool;
-        this.threadPoolMergeExecutorService = ThreadPoolMergeExecutorService.maybeCreateThreadPoolMergeExecutorService(
-            threadPool,
-            settings
-        );
         this.pluginsService = builder.pluginsService;
         this.nodeEnv = builder.nodeEnv;
         this.parserConfig = XContentParserConfiguration.EMPTY.withDeprecationHandler(LoggingDeprecationHandler.INSTANCE)
@@ -315,6 +311,12 @@ protected void doStart() {
         this.bigArrays = builder.bigArrays;
         this.scriptService = builder.scriptService;
         this.clusterService = builder.clusterService;
+        this.threadPoolMergeExecutorService = ThreadPoolMergeExecutorService.maybeCreateThreadPoolMergeExecutorService(
+            threadPool,
+            clusterService.getClusterSettings(),
+            nodeEnv
+        );
+        this.projectResolver = builder.projectResolver;
         this.client = builder.client;
         this.featureService = builder.featureService;
         this.idFieldDataEnabled = INDICES_ID_FIELD_DATA_ENABLED_SETTING.get(clusterService.getSettings());
@@ -362,7 +364,8 @@ public void onRemoval(ShardId shardId, String fieldName, boolean wasEvicted, lon
                     indicesFieldDataCache,
                     cacheCleaner,
                     indicesRequestCache,
-                    indicesQueryCache
+                    indicesQueryCache,
+                    threadPoolMergeExecutorService
                 );
             } catch (IOException e) {
                 throw new UncheckedIOException(e);

server/src/main/java/org/elasticsearch/indices/IndicesService.java

@@ -192,13 +192,17 @@ public void setUp() throws Exception {
             emptyMap()
         );
         threadPool = new TestThreadPool("test");
-        threadPoolMergeExecutorService = ThreadPoolMergeExecutorService.maybeCreateThreadPoolMergeExecutorService(threadPool, settings);
         circuitBreakerService = new NoneCircuitBreakerService();
         PageCacheRecycler pageCacheRecycler = new PageCacheRecycler(settings);
         bigArrays = new BigArrays(pageCacheRecycler, circuitBreakerService, CircuitBreaker.REQUEST);
         scriptService = new ScriptService(settings, Collections.emptyMap(), Collections.emptyMap(), () -> 1L);
-        clusterService = ClusterServiceUtils.createClusterService(threadPool);
+        clusterService = ClusterServiceUtils.createClusterService(threadPool, ClusterSettings.createBuiltInClusterSettings(settings));
         nodeEnvironment = new NodeEnvironment(settings, environment);
+        threadPoolMergeExecutorService = ThreadPoolMergeExecutorService.maybeCreateThreadPoolMergeExecutorService(
+            threadPool,
+            clusterService.getClusterSettings(),
+            nodeEnvironment
+        );
         mapperRegistry = new IndicesModule(Collections.emptyList()).getMapperRegistry();
         indexNameExpressionResolver = TestIndexNameExpressionResolver.newInstance(threadPool.getThreadContext());
     }