Merge pull request ClickHouse#63401 from ClickHouse/remove-copy-when-…

…writing-to-cache

Remove copy from writing to fs cache

src/Disks/IO/CachedOnDiskWriteBufferFromFile.cpp

@@ -41,7 +41,7 @@ FileSegmentRangeWriter::FileSegmentRangeWriter(
 {
 }
 
-bool FileSegmentRangeWriter::write(const char * data, size_t size, size_t offset, FileSegmentKind segment_kind)
+bool FileSegmentRangeWriter::write(char * data, size_t size, size_t offset, FileSegmentKind segment_kind)
 {
     if (finalized)
         return false;

src/Disks/IO/CachedOnDiskWriteBufferFromFile.h

@@ -39,7 +39,7 @@ class FileSegmentRangeWriter
     * Write a range of file segments. Allocate file segment of `max_file_segment_size` and write to
     * it until it is full and then allocate next file segment.
     */
-    bool write(const char * data, size_t size, size_t offset, FileSegmentKind segment_kind);
+    bool write(char * data, size_t size, size_t offset, FileSegmentKind segment_kind);
 
     void finalize();
 

src/Interpreters/Cache/FileSegment.cpp

@@ -340,7 +340,7 @@ void FileSegment::setRemoteFileReader(RemoteFileReaderPtr remote_file_reader_)
     remote_file_reader = remote_file_reader_;
 }
 
-void FileSegment::write(const char * from, size_t size, size_t offset)
+void FileSegment::write(char * from, size_t size, size_t offset)
 {
     ProfileEventTimeIncrement<Microseconds> watch(ProfileEvents::FileSegmentWriteMicroseconds);
 
@@ -389,16 +389,20 @@ void FileSegment::write(const char * from, size_t size, size_t offset)
 
     try
     {
-        if (!cache_writer)
-            cache_writer = std::make_unique<WriteBufferFromFile>(file_segment_path);
-
 #ifdef ABORT_ON_LOGICAL_ERROR
         /// This mutex is only needed to have a valid assertion in assertCacheCorrectness(),
         /// which is only executed in debug/sanitizer builds (under ABORT_ON_LOGICAL_ERROR).
         std::lock_guard lock(write_mutex);
 #endif
 
-        cache_writer->write(from, size);
+        if (!cache_writer)
+            cache_writer = std::make_unique<WriteBufferFromFile>(file_segment_path, /* buf_size */0);
+
+        /// Size is equal to offset as offset for write buffer points to data end.
+        cache_writer->set(from, size, /* offset */size);
+        /// Reset the buffer when finished.
+        SCOPE_EXIT({ cache_writer->set(nullptr, 0); });
+        /// Flush the buffer.
         cache_writer->next();
 
         downloaded_size += size;

src/Interpreters/Cache/FileSegment.h

@@ -204,7 +204,7 @@ friend class FileCache; /// Because of reserved_size in tryReserve().
     bool reserve(size_t size_to_reserve, size_t lock_wait_timeout_milliseconds, FileCacheReserveStat * reserve_stat = nullptr);
 
     /// Write data into reserved space.
-    void write(const char * from, size_t size, size_t offset);
+    void write(char * from, size_t size, size_t offset);
 
     // Invariant: if state() != DOWNLOADING and remote file reader is present, the reader's
     // available() == 0, and getFileOffsetOfBufferEnd() == our getCurrentWriteOffset().