Java – Really force file sync/flush in Java


How can data written to a file really be flushed/synced with the block device by Java.

I tried this code with NIO:

FileOutputStream s = new FileOutputStream(filename)
Channel c = s.getChannel()

I supposed that c.force(true) togehter with s.getFD().sync() should be sufficient because the doc for force states

Forces any updates to this channel's file to be written to the storage device that contains it.

If this channel's file resides on a local storage device then when this method returns it is guaranteed that all changes made to the file since this channel was created, or since this method was last invoked, will have been written to that device. This is useful for ensuring that critical information is not lost in the event of a system crash.

The documentation to sync states:

Force all system buffers to synchronize with the underlying device. This method returns after all modified data and attributes of this FileDescriptor have been written to the relevant device(s). In particular, if this FileDescriptor refers to a physical storage medium, such as a file in a file system, sync will not return until all in-memory modified copies of buffers associated with this FileDesecriptor have been written to the physical medium. sync is meant to be used by code that requires physical storage (such as a file) to be in a known state.

These two calls should be sufficient. Is it? I guess they aren't.

Background: I do a small performance comparison (2 GB, sequential write) using C/Java and the Java version is twice as fast as the C version and probably faster than the hardware (120 MB/s on a single HD). I also tried to execute the command line tool sync with Runtime.getRuntime().exec("sync") but that hasn't changed the behavior.

The C code resulting in 70 MB/s is (using the low level APIs (open,write,close) doesn't change much):

FILE* fp = fopen(filename, "w");
while(xyz) {
    fwrite(buffer, 1, BLOCK_SIZE, fp);

Without the final call to sync; I got unrealistical values (over 1 GB aka main memory performance).

Why is there such a big difference between C and Java? There are two possiblities: I doesn't sync the data correctly in Java or the C code is suboptimal for some reason.

I have done strace runs with "strace -cfT cmd". Here are the results:

C (Low-Level API):
MB/s 67.389782

% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 87.21    0.200012      200012         1           fdatasync
 11.05    0.025345           1     32772           write
  1.74    0.004000        4000         1           sync

C (High-Level API):
MB/s 61.796458

% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 73.19    0.144009      144009         1           sync
 26.81    0.052739           1       65539           write

Java (1.6 SUN JRE, API):
MB/s 128.6755466197537

% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 80.07  105.387609        3215     32776           write
  2.58    3.390060        3201      1059           read
  0.62    0.815251      815251         1           fsync

Java (1.6 SUN JRE, java.nio API):
MB/s 127.45830221558376

  5.52    0.980061      490031         2           fsync
  1.60    0.284752           9     32774           write
  0.00    0.000000           0        80           close

The time values seem to be system time only and are therefore pretty meaningless.

Update 2:
I switched to another server, rebooted, and I use a fresh formatted ext3. Now I get only 4% differences between Java and C. I simply don't know what went wrong. Sometimes things are strange. I should have tried the measurement with another system before writing this question. Sorry.

Update 3:
To summarize the answers:

  • Use c.force(true) followed by s.getFD().sync() for Java NIO and s.flush() and s.getFD().sync() for Java's stream API. For the High-Level API in C don't forget to sync. A fflush submitted the data to the OS, but doesn't bring your data to the block device.
  • Use strace to analyze the syscalls done by a command
  • Cross check your results before posting a question.

Update 4:
Please note the following follow-up question.

Best Solution

Actually, in C you want to just call fsync() on the one file descriptor, not sync() (or the "sync" command) which signals the kernel to flush all buffers to disk system-wide.

If you strace (getting Linux-specific here) the JVM you should be able to observe an fsync() or fdatasync() system call being made on your output file. That would be what I'd expect the getFD().sync() call to do. I assume c.force(true) simply flags to NIO that fsync() should be called after each write. It might simply be that the JVM you're using doesn't actually implement the sync() call?

I'm not sure why you weren't seeing any difference when calling "sync" as a command: but obviously, after the first sync invocation, subsequent ones are usually quite a lot faster. Again, I'd be inclined to break out strace (truss on Solaris) as a "what's actually happening here?" tool.