SERVER-42037 Add freeListSize collection stat with scale support

5 files changed, 71 insertions, 0 deletions

diff --git a/jstests/noPassthroughWithMongod/free_storage_size.js b/jstests/noPassthroughWithMongod/free_storage_size.js
new file mode 100644
index 00000000000..7526e91ea04
--- /dev/null
+++ b/jstests/noPassthroughWithMongod/free_storage_size.js
@@ -0,0 +1,43 @@
+/**
+ * Tests that db.collection.stats().freeStorageSize is a non-zero value after records are
+ * inserted and then some deleted.
+ *
+ * @tags: [
+ *   # freeStorageSize is currently only supported by WT.
+ *   requires_wiredtiger
+ * ]
+ */
+
+(function() {
+"use strict";
+
+const forceCheckpoint = () => {
+    assert.commandWorked(db.fsyncLock());
+    assert.commandWorked(db.fsyncUnlock());
+};
+
+const dbName = "freeStorageSizeTest";
+const collName = "foo";
+const testDB = db.getSiblingDB(dbName);
+const coll = testDB.getCollection(collName);
+
+const kDocsToInsert = 150;
+const kDocsToRemove = kDocsToInsert / 2;
+
+// Insert docs.
+for (let i = 0; i < kDocsToInsert; i++) {
+    assert.commandWorked(coll.insert({a: i}));
+}
+
+forceCheckpoint();
+
+// Remove docs to free up space.
+assert.commandWorked(coll.remove({a: {$lt: kDocsToRemove}}));
+
+forceCheckpoint();
+
+// Check that freeStorageSize is non-zero.
+let collStats = assert.commandWorked(testDB.runCommand({collStats: collName}));
+assert(collStats.hasOwnProperty("freeStorageSize"), tojson(collStats));
+assert.gt(collStats.freeStorageSize, 0);
+})();
diff --git a/src/mongo/db/stats/storage_stats.cpp b/src/mongo/db/stats/storage_stats.cpp
index 63e5540fdaf..dff609e2afa 100644
--- a/src/mongo/db/stats/storage_stats.cpp
+++ b/src/mongo/db/stats/storage_stats.cpp
@@ -85,6 +85,11 @@ Status appendCollectionStorageStats(OperationContext* opCtx,
         static_cast<long long>(recordStore->storageSize(opCtx, result, verbose ? 1 : 0));
     result->appendNumber("storageSize", storageSize / scale);
 
+    auto freeStorageSize = static_cast<long long>(recordStore->freeStorageSize(opCtx));
+    if (freeStorageSize != 0) {
+        result->appendNumber("freeStorageSize", freeStorageSize / scale);
+    }
+
     recordStore->appendCustomStats(opCtx, result, scale);
 
     IndexCatalog* indexCatalog = collection->getIndexCatalog();
diff --git a/src/mongo/db/storage/record_store.h b/src/mongo/db/storage/record_store.h
index 5bf95d6cb36..e549507f5a7 100644
--- a/src/mongo/db/storage/record_store.h
+++ b/src/mongo/db/storage/record_store.h
@@ -254,6 +254,15 @@ public:
                                 BSONObjBuilder* extraInfo = nullptr,
                                 int infoLevel = 0) const = 0;
 
+    /**
+     * @return file bytes available for reuse
+     * A return value of zero can mean either no bytes are available, or that the real value is
+     * unknown.
+     */
+    virtual int64_t freeStorageSize(OperationContext* opCtx) const {
+        return 0;
+    }
+
     // CRUD related
 
     /**
diff --git a/src/mongo/db/storage/wiredtiger/wiredtiger_record_store.cpp b/src/mongo/db/storage/wiredtiger/wiredtiger_record_store.cpp
index e1d49b5a2ce..6388ef65ead 100644
--- a/src/mongo/db/storage/wiredtiger/wiredtiger_record_store.cpp
+++ b/src/mongo/db/storage/wiredtiger/wiredtiger_record_store.cpp
@@ -872,6 +872,18 @@ int64_t WiredTigerRecordStore::storageSize(OperationContext* opCtx,
     return size;
 }
 
+int64_t WiredTigerRecordStore::freeStorageSize(OperationContext* opCtx) const {
+    invariant(opCtx->lockState()->isReadLocked());
+
+    WiredTigerSession* session = WiredTigerRecoveryUnit::get(opCtx)->getSessionNoTxn();
+    auto result = WiredTigerUtil::getStatisticsValue(session->getSession(),
+                                                     "statistics:" + getURI(),
+                                                     "statistics=(fast)",
+                                                     WT_STAT_DSRC_BLOCK_REUSE_BYTES);
+    uassertStatusOK(result.getStatus());
+    return result.getValue();
+}
+
 // Retrieve the value from a positioned cursor.
 RecordData WiredTigerRecordStore::_getData(const WiredTigerCursor& cursor) const {
     WT_ITEM value;
diff --git a/src/mongo/db/storage/wiredtiger/wiredtiger_record_store.h b/src/mongo/db/storage/wiredtiger/wiredtiger_record_store.h
index 30985dcd465..a7f67537583 100644
--- a/src/mongo/db/storage/wiredtiger/wiredtiger_record_store.h
+++ b/src/mongo/db/storage/wiredtiger/wiredtiger_record_store.h
@@ -137,6 +137,8 @@ public:
                                 BSONObjBuilder* extraInfo = nullptr,
                                 int infoLevel = 0) const;
 
+    virtual int64_t freeStorageSize(OperationContext* opCtx) const;
+
     // CRUD related
 
     virtual bool findRecord(OperationContext* opCtx, const RecordId& id, RecordData* out) const;