Re: Triggering non-integrity writeback from userspace

[Dave Chinner <david@fromorbit.com>]

Hi Andres,

On Wed, Oct 28, 2015 at 10:27:52AM +0100, Andres Freund wrote:
> On 2015-10-25 08:39:12 +1100, Dave Chinner wrote:
....
> > Data integrity operations require related file metadata (e.g. block
> > allocation trnascations) to be forced to the journal/disk, and a
> > device cache flush issued to ensure the data is on stable storage.
> > SYNC_FILE_RANGE_WRITE does neither of these things, and hence while
> > the IO might be the same pattern as a data integrity operation, it
> > does not provide such guarantees.
> 
> Which is desired here - the actual integrity is still going to be done
> via fsync().

OK, so you require data integrity, but....

> The idea of using SYNC_FILE_RANGE_WRITE beforehand is that
> the fsync() will only have to do very little work. The language in
> sync_file_range(2) doesn't inspire enough confidence for using it as an
> actual integrity operation :/

So really you're trying to minimise the blocking/latency of fsync()?

> > You don't want to do writeback from the syscall, right? i.e. you'd
> > like to expire the inode behind the fd, and schedule background
> > writeback to run on it immediately?
> 
> Yes, that's exactly what we want. Blocking if a process has done too
> much writes is fine tho.

OK, so it's really the latency of the fsync() operation that is what
you are trying to avoid? I've been meaning to get back to a generic
implementation of an aio fsync operation:

http://oss.sgi.com/archives/xfs/2014-06/msg00214.html

Would that be a better approach to solving your need for a
non-blocking data integrity flush of a file?

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com