MB-60276: Avoid unnecessarily calling CouchKVStore::getDbFileInfo()

This method requires reading metadata from disk on Couchstore, which is
naturally very slow. We read this metadata for every STAT("all")
request, which is wasteful and can result in very long runtimes.

This patch avoids the call made in getHistoryDiskSize() under
Couchstore, as it doesn't support history in the first place.

Magma stores the file metadata in memory, so it is not affected.

Change-Id: I59f70b6227d511ff5fa055dc9d6d6cb9c4545a07
Reviewed-on: https://review.couchbase.org/c/kv_engine/+/203267
Reviewed-by: Jim Walker <[email protected]>
Tested-by: Vesko Karaganev <[email protected]>

Author: veselink1

engines/ep/src/ep_vb.cc

@@ -441,7 +441,13 @@ uint64_t EPVBucket::getHistoryDiskSize() {
         return 0;
     }
     try {
-        return shard->getRWUnderlying()->getDbFileInfo(getId()).historyDiskSize;
+        auto& kvstore = *shard->getRWUnderlying();
+        // Avoid calling getDbFileInfo when backend does not support history.
+        // Especially important for Couchstore, as the call goes to disk.
+        if (kvstore.getStorageProperties().canRetainHistory()) {
+            return kvstore.getDbFileInfo(getId()).historyDiskSize;
+        }
+        return 0;
     } catch (std::runtime_error& error) {
         EP_LOG_WARN(
                 "EPVBucket::getHistoryDiskSize: Exception caught during "

engines/ep/src/kvstore/kvstore_iface.h

@@ -435,6 +435,8 @@ class KVStoreIface {
      * is passed in as an argument. The information returned contains
      * the item count, file size and space used.
      *
+     * Note: For some backends (Couchstore), this may require going to disk.
+     *
      * @throws std::runtime_error (and subclasses) if it was not possible to
      *         obtain the DB file info.
      */