block: flush the image with fdatasync instead of a full fsync

block/src/formats/raw/engine_sync.rs

@@ -100,8 +100,9 @@ impl AsyncIo for RawSync {
     }
 
     fn fsync(&mut self, user_data: Option<u64>) -> AsyncIoResult<()> {
+        // A guest flush needs the data and the metadata to read it back, not the timestamps.
         // SAFETY: FFI call
-        let result = unsafe { libc::fsync(self.raw_file.as_raw_fd() as libc::c_int) };
+        let result = unsafe { libc::fdatasync(self.raw_file.as_raw_fd() as libc::c_int) };
         if result < 0 {
             return Err(AsyncIoError::Fsync(io::Error::last_os_error()));
         }

block/src/io/async_io/aio_data_io.rs

@@ -104,15 +104,16 @@ impl AioDataIo {
         Ok(())
     }
 
-    /// Submits an fsync operation carrying `user_data`.
+    /// Submits an fdatasync operation carrying `user_data`: a guest flush needs the data and the
+    /// metadata to read it back, not the timestamps.
     pub fn submit_fsync(&mut self, fd: RawFd, user_data: u64) -> io::Result<()> {
         if self.in_flight.contains_key(&user_data) {
             return Err(duplicate_user_data_error(user_data));
         }
 
         let mut iocb = aio::IoControlBlock {
             aio_fildes: fd.as_raw_fd() as u32,
-            aio_lio_opcode: aio::IOCB_CMD_FSYNC as u16,
+            aio_lio_opcode: aio::IOCB_CMD_FDSYNC as u16,
             aio_data: user_data,
             aio_flags: aio::IOCB_FLAG_RESFD,
             aio_resfd: self.eventfd.as_raw_fd() as u32,

block/src/io/async_io/uring_data_io.rs

@@ -201,11 +201,14 @@ impl UringDataIo {
         self.submit_kernel_entry(user_data, &opcode::Nop::new().build().user_data(user_data))
     }
 
-    /// Submits an fsync operation carrying `user_data`.
+    /// Submits an fdatasync operation carrying `user_data`: a guest flush needs the data and the
+    /// metadata to read it back, not the timestamps, and a full fsync of a file whose mtime every
+    /// write touches forces a host journal commit per flush.
     pub fn submit_fsync(&mut self, fd: RawFd, user_data: u64) -> io::Result<()> {
         self.submit_kernel_entry(
             user_data,
             &opcode::Fsync::new(types::Fd(fd))
+                .flags(types::FsyncFlags::DATASYNC)
                 .build()
                 .user_data(user_data),
         )