From ee0d2e22e6e5b61c9c0648ba6d013b2dbb916164 Mon Sep 17 00:00:00 2001
From: Tanguy Leroux <tlrx.dev@gmail.com>
Date: Thu, 11 Feb 2021 09:16:53 +0100
Subject: [PATCH] Skip unreferenced files pruning after restore for searchable
 snapshots shards (#68821) (#68845)

When a shard is restored from a snapshot there is an after
restore step during which files on disk that are not referenced
anymore by the restored segments infos file are deleted from
disk. The deletion is executed using the
Lucene.pruneUnreferencedFiles() method which opens a new
IndexWriter and closes it in order to kick off Lucene's internal
index files deleter.

This step is not necessary for searchable snapshot shards
which use the snapshot store type because such shards
report a list of files coming from a shard snapshot which
never contain unreferenced files. Skipping this step avoids
extra unnecessary work when mounting a searchable
snapshot index.
---
 .../repositories/blobstore/FileRestoreContext.java     | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/server/src/main/java/org/elasticsearch/repositories/blobstore/FileRestoreContext.java b/server/src/main/java/org/elasticsearch/repositories/blobstore/FileRestoreContext.java
index 1321c3685b1dc..d2be6818b40ef 100644
--- a/server/src/main/java/org/elasticsearch/repositories/blobstore/FileRestoreContext.java
+++ b/server/src/main/java/org/elasticsearch/repositories/blobstore/FileRestoreContext.java
@@ -32,6 +32,7 @@
 
 import static java.util.Collections.emptyMap;
 import static java.util.Collections.unmodifiableMap;
+import static org.elasticsearch.index.IndexModule.INDEX_STORE_TYPE_SETTING;
 
 /**
  * This context will execute a file restore of the lucene files. It is primarily designed to be used to
@@ -164,11 +165,14 @@ public void restore(SnapshotFiles snapshotFiles, Store store, ActionListener<Voi
     }
 
     private void afterRestore(SnapshotFiles snapshotFiles, Store store, StoreFileMetadata restoredSegmentsFile) {
-        // read the snapshot data persisted
         try {
-            Lucene.pruneUnreferencedFiles(restoredSegmentsFile.name(), store.directory());
+            final String indexStoreType = INDEX_STORE_TYPE_SETTING.get(store.indexSettings().getSettings());
+            if ("snapshot".equals(indexStoreType) == false) {
+                Lucene.pruneUnreferencedFiles(restoredSegmentsFile.name(), store.directory());
+            }
         } catch (IOException e) {
-            throw new IndexShardRestoreFailedException(shardId, "Failed to fetch index version after copying it over", e);
+            throw new IndexShardRestoreFailedException(shardId, "Failed to remove files not referenced in segment file ["
+                + restoredSegmentsFile.name() + "] after restore", e);
         }
 
         /// now, go over and clean files that are in the store, but were not in the snapshot