Re: write-behind on streaming writes

[Linus Torvalds]

On Tue, Jun 5, 2012 at 7:57 PM, Vivek Goyal <vgoyal@xxxxxxxxxx> wrote:
>
> I had expected a bigger difference as sync_file_range() is just driving
> max queue depth of 32 (total 16MB IO in flight), while flushers are
> driving queue depths up to 140 or so. So in this paritcular test, driving
> much deeper queue depths is not really helping much. (I have seen higher
> throughputs with higher queue depths in the past. Now sure why don't we
> see it here).

How did interactivity feel?

Because quite frankly, if the throughput difference is 12.5 vs 12
seconds, I suspect the interactivity thing is what dominates.

And from my memory of the interactivity different was absolutely
*huge*. Even back when I used rotational media, I basically couldn't
even notice the background write with the sync_file_range() approach.
While the regular writeback without the writebehind had absolutely
*huge* pauses if you used something like firefox that uses fsync()
etc. And starting new applications that weren't cached was noticeably
worse too - and then with sync_file_range it wasn't even all that
noticeable.

NOTE! For the real "firefox + fsync" test, I suspect you'd need to do
the writeback on the same filesystem (and obviously disk) as your home
directory is. If the big write is to another filesystem and another
disk, I think you won't see the same issues.

Admittedly, I have not really touched anything with a rotational disk
for the last few years, nor do I ever want to see those rotating
pieces of high-tech rust ever again. And maybe your SAN has so good
latency even under load that it doesn't really matter. I remember it
mattering a lot back when..

Of course, back when I did that testing and had rotational media, we
didn't have the per-bdi writeback logic with the smart speed-dependent
depths etc, so it may be that we're just so much better at writeback
these days that it's not nearly as noticeable any more.

                        Linus
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