Allow opting out of force-merging on a cloned index in ILM's searchable snapshot action

In elastic#133954, we modified ILM's searchable snapshot action to perform the
force-merge on a clone of the index with 0 replicas. This optimization
avoids performing the force-merge redundantly on replicas, as the
subsequent snapshot operation only looks at primary shards.

We've seen some cases where cloning the index resulted in issues; there
was a bug in the clone API that caused shards to be initializing
permanently under specific circumstances (fixed by elastic#137096), and cloned
shards are unable to be assigned if their source lives on a node that is
close/past the low watermark disk threshold (will be fixed soon by the
Distributed Coordination team).

Therefore, we implement an opt-out flag that users can configure in the
`searchable_snapshot` action of their ILM policy if they don't want to
clone the index with 0 replicas before performing the force-merge. We
implement an opt-out instead of an opt-in, as we believe these issues to
be rather specific (and soon resolved), and the clone is worth doing by
default.

Author: nielsbauman

docs/reference/elasticsearch/index-lifecycle-actions/ilm-searchable-snapshot.md

@@ -46,8 +46,11 @@ By default, this snapshot is deleted by the [delete action](/reference/elasticse
 
     This force merging occurs in the phase that the index is in **prior** to the `searchable_snapshot` action. For example, if using a `searchable_snapshot` action in the `hot` phase, the force merge will be performed on the hot nodes. If using a `searchable_snapshot` action in the `cold` phase, the force merge will be performed on whatever tier the index is **prior** to the `cold` phase (either `hot` or `warm`).
 
+`force_merge_on_clone`
+:   (Optional, Boolean) By default, if `force_merge_index` is `true`, the index will first be cloned with 0 replicas and the force-merge will be performed on the clone before the searchable snapshot is created. This avoids performing the force-merge redundantly on replica shards, as the snapshot operation only uses primary shards. Setting this option to `false` will skip the clone step and perform the force-merge directly on the managed index. Defaults to `true`.
+
 `total_shards_per_node`
-:   The maximum number of shards (replicas and primaries) that will be allocated to a single node for the searchable snapshot index. Defaults to unbounded.
+:   (Optional, Integer) The maximum number of shards (replicas and primaries) that will be allocated to a single node for the searchable snapshot index. Defaults to unbounded.
 
 
 ## Examples [ilm-searchable-snapshot-ex]

server/src/main/resources/transport/definitions/referable/ilm_searchable_snapshot_opt_out_clone.csv

@@ -0,0 +1 @@
+9206000

server/src/main/resources/transport/upper_bounds/9.3.csv

@@ -1 +1 @@
-text_similarity_rank_docs_explain_chunks,9205000
+ilm_searchable_snapshot_opt_out_clone,9206000

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ilm/SearchableSnapshotAction.java

@@ -8,6 +8,7 @@
 
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
+import org.elasticsearch.TransportVersion;
 import org.elasticsearch.TransportVersions;
 import org.elasticsearch.action.admin.indices.shrink.ResizeType;
 import org.elasticsearch.client.internal.Client;
@@ -58,6 +59,12 @@ public class SearchableSnapshotAction implements LifecycleAction {
     public static final ParseField FORCE_MERGE_INDEX = new ParseField("force_merge_index");
     public static final ParseField TOTAL_SHARDS_PER_NODE = new ParseField("total_shards_per_node");
     public static final ParseField REPLICATE_FOR = new ParseField("replicate_for");
+    public static final ParseField FORCE_MERGE_ON_CLONE = new ParseField("force_merge_on_clone");
+
+    private static final TransportVersion FORCE_MERGE_ON_CLONE_TRANSPORT_VERSION = TransportVersion.fromName(
+        "ilm_searchable_snapshot_opt_out_clone"
+    );
+
     public static final String CONDITIONAL_SKIP_ACTION_STEP = BranchingStep.NAME + "-check-prerequisites";
     public static final String CONDITIONAL_SKIP_GENERATE_AND_CLEAN = BranchingStep.NAME + "-check-existing-snapshot";
     public static final String CONDITIONAL_SKIP_CLONE_STEP = BranchingStep.NAME + "-skip-clone-check";
@@ -87,7 +94,7 @@ public class SearchableSnapshotAction implements LifecycleAction {
 
     private static final ConstructingObjectParser<SearchableSnapshotAction, Void> PARSER = new ConstructingObjectParser<>(
         NAME,
-        a -> new SearchableSnapshotAction((String) a[0], a[1] == null || (boolean) a[1], (Integer) a[2], (TimeValue) a[3])
+        a -> new SearchableSnapshotAction((String) a[0], a[1] == null || (boolean) a[1], (Integer) a[2], (TimeValue) a[3], (Boolean) a[4])
     );
 
     static {
@@ -100,6 +107,7 @@ public class SearchableSnapshotAction implements LifecycleAction {
             REPLICATE_FOR,
             ObjectParser.ValueType.STRING
         );
+        PARSER.declareBoolean(ConstructingObjectParser.optionalConstructorArg(), FORCE_MERGE_ON_CLONE);
     }
 
     public static SearchableSnapshotAction parse(XContentParser parser) {
@@ -112,12 +120,16 @@ public static SearchableSnapshotAction parse(XContentParser parser) {
     private final Integer totalShardsPerNode;
     @Nullable
     private final TimeValue replicateFor;
+    /** Opt-out field for forcing the force-merge step to run on the source index instead of a cloned version with 0 replicas. */
+    @Nullable
+    private final Boolean forceMergeOnClone;
 
     public SearchableSnapshotAction(
         String snapshotRepository,
         boolean forceMergeIndex,
         @Nullable Integer totalShardsPerNode,
-        @Nullable TimeValue replicateFor
+        @Nullable TimeValue replicateFor,
+        @Nullable Boolean forceMergeOnClone
     ) {
         if (Strings.hasText(snapshotRepository) == false) {
             throw new IllegalArgumentException("the snapshot repository must be specified");
@@ -136,21 +148,31 @@ public SearchableSnapshotAction(
             );
         }
         this.replicateFor = replicateFor;
+
+        if (forceMergeIndex == false && forceMergeOnClone != null) {
+            throw new IllegalArgumentException(
+                "[" + FORCE_MERGE_ON_CLONE.getPreferredName() + "] is not allowed when [" + FORCE_MERGE_INDEX + "] is [false]"
+            );
+        }
+        this.forceMergeOnClone = forceMergeOnClone;
     }
 
     public SearchableSnapshotAction(String snapshotRepository, boolean forceMergeIndex) {
-        this(snapshotRepository, forceMergeIndex, null, null);
+        this(snapshotRepository, forceMergeIndex, null, null, null);
     }
 
     public SearchableSnapshotAction(String snapshotRepository) {
-        this(snapshotRepository, true, null, null);
+        this(snapshotRepository, true, null, null, null);
     }
 
     public SearchableSnapshotAction(StreamInput in) throws IOException {
         this.snapshotRepository = in.readString();
         this.forceMergeIndex = in.readBoolean();
         this.totalShardsPerNode = in.getTransportVersion().onOrAfter(TransportVersions.V_8_16_0) ? in.readOptionalInt() : null;
         this.replicateFor = in.getTransportVersion().supports(TransportVersions.V_8_18_0) ? in.readOptionalTimeValue() : null;
+        this.forceMergeOnClone = in.getTransportVersion().supports(FORCE_MERGE_ON_CLONE_TRANSPORT_VERSION)
+            ? in.readOptionalBoolean()
+            : null;
     }
 
     public boolean isForceMergeIndex() {
@@ -171,6 +193,11 @@ public TimeValue getReplicateFor() {
         return replicateFor;
     }
 
+    @Nullable
+    public Boolean isForceMergeOnClone() {
+        return forceMergeOnClone;
+    }
+
     @Override
     public List<Step> toSteps(Client client, String phase, StepKey nextStepKey) {
         assert false;
@@ -299,9 +326,12 @@ public List<Step> toSteps(Client client, String phase, StepKey nextStepKey, XPac
             Instant::now
         );
 
+        // We force-merge on the clone by default, but allow the user to opt-out of this behavior if there is any reason why they don't want
+        // to clone the index (e.g. if something is preventing the cloned index shards from being assigned).
+        StepKey keyForForceMerge = shouldForceMergeOnClone() ? conditionalSkipCloneKey : forceMergeStepKey;
         // When generating a snapshot, we either jump to the force merge section, or we skip the
         // forcemerge and go straight to steps for creating the snapshot
-        StepKey keyForSnapshotGeneration = forceMergeIndex ? conditionalSkipCloneKey : generateSnapshotNameKey;
+        StepKey keyForSnapshotGeneration = forceMergeIndex ? keyForForceMerge : generateSnapshotNameKey;
         // Branch, deciding whether there is an existing searchable snapshot that can be used for mounting the index
         // (in which case, skip generating a new name and the snapshot cleanup), or if we need to generate a new snapshot
         BranchingStep skipGeneratingSnapshotStep = new BranchingStep(
@@ -529,12 +559,14 @@ public List<Step> toSteps(Client client, String phase, StepKey nextStepKey, XPac
         steps.add(waitUntilTimeSeriesEndTimeStep);
         steps.add(skipGeneratingSnapshotStep);
         if (forceMergeIndex) {
-            steps.add(conditionalSkipCloneStep);
-            steps.add(readOnlyStep);
-            steps.add(cleanupClonedIndexStep);
-            steps.add(generateCloneIndexNameStep);
-            steps.add(cloneIndexStep);
-            steps.add(waitForClonedIndexGreenStep);
+            if (shouldForceMergeOnClone()) {
+                steps.add(conditionalSkipCloneStep);
+                steps.add(readOnlyStep);
+                steps.add(cleanupClonedIndexStep);
+                steps.add(generateCloneIndexNameStep);
+                steps.add(cloneIndexStep);
+                steps.add(waitForClonedIndexGreenStep);
+            }
             steps.add(forceMergeStep);
             steps.add(segmentCountStep);
         }
@@ -581,6 +613,15 @@ static MountSearchableSnapshotRequest.Storage getConcreteStorageType(StepKey cur
         }
     }
 
+    /**
+     * Returns whether we should first clone the index and perform the force-merge on that cloned index (true) or force-merge on the
+     * original index (false). Defaults to true when {@link #forceMergeOnClone} is null/unspecified. Note that this value is ignored when
+     * {@link #forceMergeIndex} is false.
+     */
+    private boolean shouldForceMergeOnClone() {
+        return forceMergeOnClone == null || forceMergeOnClone;
+    }
+
     @Override
     public boolean isSafeAction() {
         return true;
@@ -601,6 +642,9 @@ public void writeTo(StreamOutput out) throws IOException {
         if (out.getTransportVersion().supports(TransportVersions.V_8_18_0)) {
             out.writeOptionalTimeValue(replicateFor);
         }
+        if (out.getTransportVersion().supports(FORCE_MERGE_ON_CLONE_TRANSPORT_VERSION)) {
+            out.writeOptionalBoolean(forceMergeOnClone);
+        }
     }
 
     @Override
@@ -614,6 +658,9 @@ public XContentBuilder toXContent(XContentBuilder builder, Params params) throws
         if (replicateFor != null) {
             builder.field(REPLICATE_FOR.getPreferredName(), replicateFor);
         }
+        if (forceMergeOnClone != null) {
+            builder.field(FORCE_MERGE_ON_CLONE.getPreferredName(), forceMergeOnClone);
+        }
         builder.endObject();
         return builder;
     }
@@ -630,12 +677,13 @@ public boolean equals(Object o) {
         return Objects.equals(snapshotRepository, that.snapshotRepository)
             && Objects.equals(forceMergeIndex, that.forceMergeIndex)
             && Objects.equals(totalShardsPerNode, that.totalShardsPerNode)
-            && Objects.equals(replicateFor, that.replicateFor);
+            && Objects.equals(replicateFor, that.replicateFor)
+            && Objects.equals(forceMergeOnClone, that.forceMergeOnClone);
     }
 
     @Override
     public int hashCode() {
-        return Objects.hash(snapshotRepository, forceMergeIndex, totalShardsPerNode, replicateFor);
+        return Objects.hash(snapshotRepository, forceMergeIndex, totalShardsPerNode, replicateFor, forceMergeOnClone);
     }
 
     @Nullable

x-pack/plugin/core/src/test/java/org/elasticsearch/xpack/core/ilm/LifecyclePolicyTests.java

@@ -222,6 +222,7 @@ public static LifecyclePolicy randomTimeseriesLifecyclePolicy(@Nullable String l
                             randomAlphaOfLength(10),
                             randomBoolean(),
                             (randomBoolean() ? null : randomIntBetween(1, 100)),
+                            null,
                             null
                         )
                     )

x-pack/plugin/core/src/test/java/org/elasticsearch/xpack/core/ilm/SearchableSnapshotActionTests.java

@@ -37,7 +37,12 @@ public void testToSteps() {
 
         List<Step> steps = action.toSteps(null, phase, nextStepKey, null);
 
-        List<StepKey> expectedSteps = expectedStepKeys(phase, action.isForceMergeIndex(), action.getReplicateFor() != null);
+        List<StepKey> expectedSteps = expectedStepKeys(
+            phase,
+            action.isForceMergeIndex(),
+            action.getReplicateFor() != null,
+            action.isForceMergeOnClone()
+        );
 
         assertThat(steps.size(), is(expectedSteps.size()));
         for (int i = 0; i < expectedSteps.size(); i++) {
@@ -54,7 +59,7 @@ public void testToSteps() {
         CreateSnapshotStep createSnapshotStep = (CreateSnapshotStep) steps.get(index);
         assertThat(createSnapshotStep.getNextKeyOnIncomplete(), is(expectedSteps.get(index - 1)));
         validateWaitForDataTierStep(phase, steps, index + 1, index + 2);
-        validateForceMergeClone(action.isForceMergeIndex(), steps);
+        validateForceMergeClone(action.isForceMergeIndex(), action.isForceMergeOnClone(), steps);
     }
 
     private void validateWaitForDataTierStep(String phase, List<Step> steps, int waitForDataTierStepIndex, int mountStepIndex) {
@@ -70,8 +75,8 @@ private void validateWaitForDataTierStep(String phase, List<Step> steps, int wai
     /**
      * Validate that the {@link ResizeIndexStep} used to clone the index for force merging configures the target index with 0 replicas.
      */
-    private void validateForceMergeClone(boolean isForceMergeIndex, List<Step> steps) {
-        if (isForceMergeIndex == false) {
+    private void validateForceMergeClone(boolean isForceMergeIndex, Boolean isForceMergeOnClone, List<Step> steps) {
+        if (isForceMergeIndex == false || (isForceMergeOnClone != null && isForceMergeOnClone == false)) {
             return;
         }
         ResizeIndexStep cloneStep = (ResizeIndexStep) steps.stream()
@@ -114,24 +119,25 @@ public void testCreateWithInvalidTotalShardsPerNode() {
 
         IllegalArgumentException exception = expectThrows(
             IllegalArgumentException.class,
-            () -> new SearchableSnapshotAction("test", true, invalidTotalShardsPerNode, null)
+            () -> new SearchableSnapshotAction("test", true, invalidTotalShardsPerNode, null, null)
         );
         assertEquals("[" + TOTAL_SHARDS_PER_NODE.getPreferredName() + "] must be >= 1", exception.getMessage());
     }
 
-    private List<StepKey> expectedStepKeys(String phase, boolean forceMergeIndex, boolean hasReplicateFor) {
+    private List<StepKey> expectedStepKeys(String phase, boolean forceMergeIndex, boolean hasReplicateFor, Boolean forceMergeOnClone) {
+        final var shouldForceMergeOnClone = forceMergeOnClone != null ? forceMergeOnClone : forceMergeIndex;
         return Stream.of(
             new StepKey(phase, NAME, SearchableSnapshotAction.CONDITIONAL_SKIP_ACTION_STEP),
             new StepKey(phase, NAME, CheckNotDataStreamWriteIndexStep.NAME),
             new StepKey(phase, NAME, WaitForNoFollowersStep.NAME),
             new StepKey(phase, NAME, WaitUntilTimeSeriesEndTimePassesStep.NAME),
             new StepKey(phase, NAME, SearchableSnapshotAction.CONDITIONAL_SKIP_GENERATE_AND_CLEAN),
-            forceMergeIndex ? new StepKey(phase, NAME, SearchableSnapshotAction.CONDITIONAL_SKIP_CLONE_STEP) : null,
-            forceMergeIndex ? new StepKey(phase, NAME, ReadOnlyStep.NAME) : null,
-            forceMergeIndex ? new StepKey(phase, NAME, CleanupGeneratedIndexStep.NAME) : null,
-            forceMergeIndex ? new StepKey(phase, NAME, GenerateUniqueIndexNameStep.NAME) : null,
-            forceMergeIndex ? new StepKey(phase, NAME, ResizeIndexStep.CLONE) : null,
-            forceMergeIndex ? new StepKey(phase, NAME, SearchableSnapshotAction.WAIT_FOR_CLONED_INDEX_GREEN) : null,
+            shouldForceMergeOnClone ? new StepKey(phase, NAME, SearchableSnapshotAction.CONDITIONAL_SKIP_CLONE_STEP) : null,
+            shouldForceMergeOnClone ? new StepKey(phase, NAME, ReadOnlyStep.NAME) : null,
+            shouldForceMergeOnClone ? new StepKey(phase, NAME, CleanupGeneratedIndexStep.NAME) : null,
+            shouldForceMergeOnClone ? new StepKey(phase, NAME, GenerateUniqueIndexNameStep.NAME) : null,
+            shouldForceMergeOnClone ? new StepKey(phase, NAME, ResizeIndexStep.CLONE) : null,
+            shouldForceMergeOnClone ? new StepKey(phase, NAME, SearchableSnapshotAction.WAIT_FOR_CLONED_INDEX_GREEN) : null,
             forceMergeIndex ? new StepKey(phase, NAME, ForceMergeStep.NAME) : null,
             forceMergeIndex ? new StepKey(phase, NAME, SegmentCountStep.NAME) : null,
             new StepKey(phase, NAME, GenerateSnapshotNameStep.NAME),
@@ -170,43 +176,46 @@ protected Writeable.Reader<SearchableSnapshotAction> instanceReader() {
 
     @Override
     protected SearchableSnapshotAction mutateInstance(SearchableSnapshotAction instance) {
-        return switch (randomIntBetween(0, 3)) {
-            case 0 -> new SearchableSnapshotAction(
-                randomAlphaOfLengthBetween(5, 10),
-                instance.isForceMergeIndex(),
-                instance.getTotalShardsPerNode(),
-                instance.getReplicateFor()
-            );
-            case 1 -> new SearchableSnapshotAction(
-                instance.getSnapshotRepository(),
-                instance.isForceMergeIndex() == false,
-                instance.getTotalShardsPerNode(),
-                instance.getReplicateFor()
-            );
-            case 2 -> new SearchableSnapshotAction(
-                instance.getSnapshotRepository(),
-                instance.isForceMergeIndex(),
-                instance.getTotalShardsPerNode() == null ? 1 : instance.getTotalShardsPerNode() + randomIntBetween(1, 100),
-                instance.getReplicateFor()
-            );
-            case 3 -> new SearchableSnapshotAction(
-                instance.getSnapshotRepository(),
-                instance.isForceMergeIndex(),
-                instance.getTotalShardsPerNode(),
-                instance.getReplicateFor() == null
-                    ? TimeValue.timeValueDays(1)
-                    : TimeValue.timeValueDays(instance.getReplicateFor().getDays() + randomIntBetween(1, 10))
-            );
+        var snapshotRepository = instance.getSnapshotRepository();
+        var forceMergeIndex = instance.isForceMergeIndex();
+        var totalShardsPerNode = instance.getTotalShardsPerNode();
+        var replicateFor = instance.getReplicateFor();
+        var forceMergeOnClone = instance.isForceMergeOnClone();
+        switch (randomIntBetween(0, 4)) {
+            case 0 -> snapshotRepository = randomAlphaOfLengthBetween(5, 10);
+            case 1 -> {
+                forceMergeIndex = forceMergeIndex == false;
+                if (forceMergeIndex == false) {
+                    forceMergeOnClone = null;
+                }
+            }
+            case 2 -> totalShardsPerNode = totalShardsPerNode == null ? 1 : totalShardsPerNode + randomIntBetween(1, 100);
+            case 3 -> replicateFor = replicateFor == null
+                ? TimeValue.timeValueDays(1)
+                : TimeValue.timeValueDays(replicateFor.getDays() + randomIntBetween(1, 10));
+            case 4 -> {
+                if (forceMergeOnClone == null) {
+                    forceMergeOnClone = randomBoolean();
+                } else {
+                    forceMergeOnClone = randomBoolean() ? null : forceMergeOnClone == false;
+                }
+                if (forceMergeOnClone != null) {
+                    forceMergeIndex = true;
+                }
+            }
             default -> throw new IllegalArgumentException("Invalid mutation branch");
-        };
+        }
+        return new SearchableSnapshotAction(snapshotRepository, forceMergeIndex, totalShardsPerNode, replicateFor, forceMergeOnClone);
     }
 
     static SearchableSnapshotAction randomInstance() {
+        final var forceMergeIndex = randomBoolean();
         return new SearchableSnapshotAction(
             randomAlphaOfLengthBetween(5, 10),
-            randomBoolean(),
+            forceMergeIndex,
             (randomBoolean() ? null : randomIntBetween(1, 100)),
-            (randomBoolean() ? null : TimeValue.timeValueDays(randomIntBetween(1, 10)))
+            (randomBoolean() ? null : TimeValue.timeValueDays(randomIntBetween(1, 10))),
+            forceMergeIndex && randomBoolean() ? randomBoolean() : null
         );
     }
 }

x-pack/plugin/core/src/test/java/org/elasticsearch/xpack/core/ilm/TimeseriesLifecycleTypeTests.java

@@ -813,7 +813,8 @@ public void testValidateReplicateFor() {
             "repo",
             randomBoolean(),
             randomBoolean() ? null : randomIntBetween(1, 100), // the ESTestCase utility can produce zeroes, which we can't have here
-            TimeValue.timeValueDays(10)
+            TimeValue.timeValueDays(10),
+            null
         );
 
         // first test case: there's a replicate_for, but it isn't on the first searchable_snapshot action