SERVER-19213 Move old snapshot cleanup out of the repl mutex

4 files changed, 27 insertions, 3 deletions

diff --git a/src/mongo/db/repl/oplog.cpp b/src/mongo/db/repl/oplog.cpp
index 7be8430c56a..2c08adab0a7 100644
--- a/src/mongo/db/repl/oplog.cpp
+++ b/src/mongo/db/repl/oplog.cpp
@@ -1019,6 +1019,9 @@ void SnapshotThread::run() {
             log() << "skipping storage snapshot pass due to write conflict";
             continue;
         }
+
+        // Called outside of all locks.
+        _manager->cleanupUnneededSnapshots();
     }
 }
 
diff --git a/src/mongo/db/storage/snapshot_manager.h b/src/mongo/db/storage/snapshot_manager.h
index e4a30199525..8d4b81fd5c3 100644
--- a/src/mongo/db/storage/snapshot_manager.h
+++ b/src/mongo/db/storage/snapshot_manager.h
@@ -71,15 +71,27 @@ public:
     virtual Status createSnapshot(OperationContext* txn, const SnapshotName& name) = 0;
 
     /**
-     * Sets the snapshot to be used for committed reads. Once set, all older snapshots that are
-     * not currently in use by any RecoveryUnit can be deleted.
+     * Sets the snapshot to be used for committed reads.
      *
      * Implementations are allowed to assume that all older snapshots have names that compare
      * less than the passed in name, and newer ones compare greater.
+     *
+     * This is called while holding a very hot mutex. Therefore it should avoid doing any work that
+     * can be done later. In particular, cleaning up of old snapshots should be deferred until
+     * cleanupUnneededSnapshots is called.
      */
     virtual void setCommittedSnapshot(const SnapshotName& name) = 0;
 
     /**
+     * Cleans up all snapshots older than the current committed snapshot.
+     *
+     * Operations that have already begun using an older snapshot must continue to work using that
+     * snapshot until they would normally start using a newer one. Any implementation that allows
+     * that without an unbounded growth of snapshots is permitted.
+     */
+    virtual void cleanupUnneededSnapshots() = 0;
+
+    /**
      * Drops all snapshots and clears the "committed" snapshot.
      */
     virtual void dropAllSnapshots() = 0;
diff --git a/src/mongo/db/storage/wiredtiger/wiredtiger_snapshot_manager.cpp b/src/mongo/db/storage/wiredtiger/wiredtiger_snapshot_manager.cpp
index 56be58ffdf3..a56f4827f59 100644
--- a/src/mongo/db/storage/wiredtiger/wiredtiger_snapshot_manager.cpp
+++ b/src/mongo/db/storage/wiredtiger/wiredtiger_snapshot_manager.cpp
@@ -57,8 +57,16 @@ void WiredTigerSnapshotManager::setCommittedSnapshot(const SnapshotName& name) {
 
     invariant(!_committedSnapshot || *_committedSnapshot <= name);
     _committedSnapshot = name;
+}
+
+void WiredTigerSnapshotManager::cleanupUnneededSnapshots() {
+    stdx::lock_guard<stdx::mutex> lock(_mutex);
+
+    if (!_committedSnapshot)
+        return;
 
-    const std::string config = str::stream() << "drop=(before=" << name.asU64() << ')';
+    const std::string config = str::stream() << "drop=(before=" << _committedSnapshot->asU64()
+                                             << ')';
     invariantWTOK(_session->snapshot(_session, config.c_str()));
 }
 
diff --git a/src/mongo/db/storage/wiredtiger/wiredtiger_snapshot_manager.h b/src/mongo/db/storage/wiredtiger/wiredtiger_snapshot_manager.h
index 956f581bded..912f0fb13d4 100644
--- a/src/mongo/db/storage/wiredtiger/wiredtiger_snapshot_manager.h
+++ b/src/mongo/db/storage/wiredtiger/wiredtiger_snapshot_manager.h
@@ -54,6 +54,7 @@ public:
     Status prepareForCreateSnapshot(OperationContext* txn) final;
     Status createSnapshot(OperationContext* ru, const SnapshotName& name) final;
     void setCommittedSnapshot(const SnapshotName& name) final;
+    void cleanupUnneededSnapshots() final;
     void dropAllSnapshots() final;
 
     //