On Fri, Dec 07, 2012 at 03:25:53PM -0800, Howard Chu wrote:

Ric Wheeler wrote:

quoted

On 12/07/2012 04:14 PM, Theodore Ts'o wrote:

quoted

On Fri, Dec 07, 2012 at 02:30:19PM -0500, Steven Rostedt wrote:

quoted

How is this similar? By adding this bit, we removed incentive from a
group of developers that have the means to fix the real issue at hand
(the performance problem with ext4). Thus, it means that they have a work
around that's good enough for them, but the rest of us suffer.

That assumes that there **is** a way to claw back the performance
loss, and Chris Mason has demonstrated the performance hit exists with
xfs as well (950 MB/s vs. 400 MB/s; that's more than a factor of two).
Sometimes, you have to make the engineering tradeoffs.  That's why
we're engineers, for goodness sakes.  Sometimes, it's just not
possible to square the circle.

I don't believe that the technique of forcing people who need that
performance to suffer in order to induce them to try to engineer a
solution which may or may not exist is really the best or fairest way
to go about things.

				- Ted

This is not a generally useful feature and won't ship in a way that helps most
users with this issue.

quoted

Let's fix the problem properly.

In the meantime, there are several obvious ways to avoid this performance hit
without changing the kernel (fully allocate and write the data, certainly
reasonable for even reasonable sized files).

I have to agree that, if this is going to be an ext4-specific
feature, then it can just be implemented via an ext4-specific ioctl
and be done with it. But I'm not convinced this should be an
ext4-specific feature.

As for "fix the problem properly" - you're fixing the wrong problem.
This type of feature is important to me, not just because of the
performance issue. As has already been pointed out, the performance
difference may even be negligible.

But on SSDs, the issue is write endurance. The whole point of
preallocating a file is to avoid doing incremental metadata updates.
Particularly when each of those 1-bit status updates costs entire
blocks, and gratuitously shortens the life of the media. The fact
that avoiding the unnecessary wear and tear may also yield a
performance boost is just icing on the cake. (And if the perf boost
is over a factor of 2:1 that's some pretty damn good icing.)

That's a filesystem implementation specific problem, not a generic
fallocate() or unwritten extent conversion problem.

Besides, ext4 doesn't write back every metadata modification that is
made - they are aggregated in memory and only written when the
journal is full or the metadata ages out. Hence unwritten extent
conversion has very little impact on the amount of writes that are
done to the flash because it is vastly dominated by the data writes.

Similarly, in XFS you might see a few thousand or tens of thousands
of metadata blocks get written once every 30s under such a random
write workload, but each metadata block might have gone through a
million changes in memory since the last time it was written.
Indeed, in that 30s, there would have been a few million random data
writes so the metadata writes are well and truly lost in the
noise...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com