File size is updated correctly according to 

> fdatasync() is similar to fsync(), but does not flush modified metadata unless that metadata is needed in order to allow a subsequent data retrieval to be correctly handled.  For example, changes to  st_atime  or  st_mtime  (respec‐
       tively,  time  of  last  access  and  time  of last modification; see inode(7)) do not require flushing because they are not necessary for a subsequent data read to be handled correctly.  On the other hand, a change to the file size
       (st_size, as made by say ftruncate(2)), would require a metadata flush.

And we don't care about modification times.
Should work alright.
#include <Disks/LocalDirectorySyncGuard.h>
#include <Common/Exception.h>
#include <Disks/IDisk.h>
#include <fcntl.h> 
#ifndef O_DIRECTORY
#define O_DIRECTORY O_RDWR
#endif
namespace DB
{
namespace ErrorCodes
{
    extern const int CANNOT_FSYNC;
    extern const int FILE_DOESNT_EXIST;
    extern const int CANNOT_OPEN_FILE;
    extern const int CANNOT_CLOSE_FILE;
}
LocalDirectorySyncGuard::LocalDirectorySyncGuard(const String & full_path)