[Opt](cloud) Using file cache disks number to determine the size of t…

…he flush memtable thread pool

be/src/cloud/cloud_storage_engine.cpp

@@ -38,6 +38,7 @@
 #include "cloud/cloud_warm_up_manager.h"
 #include "cloud/config.h"
 #include "io/cache/block_file_cache_downloader.h"
+#include "io/cache/block_file_cache_factory.h"
 #include "io/cache/file_cache_common.h"
 #include "io/fs/file_system.h"
 #include "io/fs/hdfs_file_system.h"
@@ -180,8 +181,8 @@ Status CloudStorageEngine::open() {
  // TODO(plat1ko): DeleteBitmapTxnManager
 
  _memtable_flush_executor = std::make_unique<MemTableFlushExecutor>();
- // TODO(plat1ko): Use file cache disks number?
- _memtable_flush_executor->init(1);
+ // Use file cache disks number
+ _memtable_flush_executor->init(io::FileCacheFactory::instance()->get_cache_instance_size());
 
  _calc_delete_bitmap_executor = std::make_unique<CalcDeleteBitmapExecutor>();
  _calc_delete_bitmap_executor->init();