Re: [patch resend v4] update ctime and mtime for mmaped write

Miklos Szeredi
2007-03-27T18:56:37Z

> > Yes, this will make msync(MS_ASYNC) more heavyweight again. But if an
> > application doesn't want to update the timestamps, it should just omit
> > this call, since it does nothing else.
>
> Er... FWIW, I have an application that makes heavy use of msync(MS_ASYNC)
> and doesn't care about timestamps. (In fact, sometimes it's configured
> to write to a raw device and there are no timestamps.)
>
> It's used as a poor man's portable async I/O. The application logs
> data to disk, and sometimes needs to sync it to disk to ensure it has
> all been written.
>
> To reduce long pauses when doing msync(MS_SYNC), it does msync(MS_ASYNC)
> as soon as a page is filled up to prompt asynchronous writeback.
> "I'm done writing this page and don't intend to write it again.
> Please start committing it to stable storage, but don't block me."
>
> Then, occasionally, there's an msync(MS_SYNC) call to be sure the data
> is synced to disk. This caused annoying hiccups before the MS_ASYNC
> calls were added.
>
>
> I agree that msync(MS_ASYNC) has no semantics if time is ignored.
> But it's a useful way to tell the OS that the page is not going
> to be dirtied again.

Just to clarify, here's the header comment for sys_msync():

/*
* MS_SYNC syncs the entire file - including mappings.
*
* MS_ASYNC does not start I/O (it used to, up to 2.5.67).
* Nor does it marks the relevant pages dirty (it used to up to 2.6.17).
* Now it doesn't do anything, since dirty pages are properly tracked.
*
* The application may now run fsync() to
* write out the dirty pages and wait on the writeout and check the result.
* Or the application may run fadvise(FADV_DONTNEED) against the fd to start
* async writeout immediately.
* So by _not_ starting I/O in MS_ASYNC we provide complete flexibility to
* applications.
*/

It's actually wrong about FADV_DONTNEED, which I think doesn't start
writeout either. So there you have it ;)

Miklos