From 6f9f7d118da12c9988d98f7a68db18800a04d903 Mon Sep 17 00:00:00 2001
From: Alexey Milovidov <milovidov@yandex-team.ru>
Date: Sun, 28 Mar 2021 04:27:26 +0300
Subject: [PATCH] Fixes

---
 programs/server/config.xml               | 6 ++++--
 src/Storages/MergeTree/MergeTreeData.cpp | 3 ++-
 2 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/programs/server/config.xml b/programs/server/config.xml
index 543f7c54ca6..50c29e05a4c 100644
--- a/programs/server/config.xml
+++ b/programs/server/config.xml
@@ -297,20 +297,22 @@
       -->
     <mark_cache_size>5368709120</mark_cache_size>
 
+
     <!-- If you enable the `min_bytes_to_use_mmap_io` setting,
          the data in MergeTree tables can be read with mmap to avoid copying from kernel to userspace.
          It makes sense only for large files and helps only if data reside in page cache.
          To avoid frequent open/mmap/munmap/close calls (which are very expensive due to consequent page faults)
          and to reuse mappings from several threads and queries,
          the cache of mapped files is maintained. Its size is the number of mapped regions (usually equal to the number of mapped files).
-         The amount of data in mapped files can be monitored in system.metrics, system.metric_log by the MappedFiles, MappedFileBytes metrics
+         The amount of data in mapped files can be monitored
+         in system.metrics, system.metric_log by the MappedFiles, MappedFileBytes metrics
          and in system.asynchronous_metrics, system.asynchronous_metrics_log by the MMapCacheCells metric,
          and also in system.events, system.processes, system.query_log, system.query_thread_log by the
          CreatedReadBufferMMap, CreatedReadBufferMMapFailed, MappedFileCacheHits, MappedFileCacheMisses events.
          Note that the amount of data in mapped files does not consume memory directly and is not accounted
          in query or server memory usage - because this memory can be discarded similar to OS page cache.
          The cache is dropped (the files are closed) automatically on removal of old parts in MergeTree,
-         also it can be dropped manually by the SYSTEM DROP MMAP CACHE query. Good luck.
+         also it can be dropped manually by the SYSTEM DROP MMAP CACHE query.
       -->
     <mmap_cache_size>1000</mmap_cache_size>
 
diff --git a/src/Storages/MergeTree/MergeTreeData.cpp b/src/Storages/MergeTree/MergeTreeData.cpp
index 039b9281135..f5007918a22 100644
--- a/src/Storages/MergeTree/MergeTreeData.cpp
+++ b/src/Storages/MergeTree/MergeTreeData.cpp
@@ -1163,7 +1163,8 @@ void MergeTreeData::clearOldPartsFromFilesystem(bool force)
 
     /// This is needed to close files to avoid they reside on disk after being deleted.
     /// NOTE: we can drop files from cache more selectively but this is good enough.
-    global_context.dropMappedFileCache();
+    if (!parts_to_remove.empty())
+        global_context.dropMappedFileCache();
 }
 
 void MergeTreeData::clearPartsFromFilesystem(const DataPartsVector & parts_to_remove)