Re: xfs: does mkfs.xfs require fancy switches to get decent performance? (was Tux3 Report: How fast can we fsync?)

[Christian Stroetmann]

On the 30th of April 2015 17:14, Daniel Phillips wrote:

Hallo hardcore coders

> On 04/30/2015 07:28 AM, Howard Chu wrote:
> > Daniel Phillips wrote:
> > > On 04/30/2015 06:48 AM, Mike Galbraith wrote:
> > > > On Thu, 2015-04-30 at 05:58 -0700, Daniel Phillips wrote:
> > > > > On Thursday, April 30, 2015 5:07:21 AM PDT, Mike Galbraith wrote:
> > > > > > On Thu, 2015-04-30 at 04:14 -0700, Daniel Phillips wrote:
> > > > > >
> > > > > > > Lovely sounding argument, but it is wrong because Tux3 still beats XFS
> > > > > > > even with seek time factored out of the equation.
> > > > > > Hm.  Do you have big-storage comparison numbers to back that?  I'm no
> > > > > > storage guy (waiting for holographic crystal arrays to obsolete all this
> > > > > > crap;), but Dave's big-storage guy words made sense to me.
> > > > > This has nothing to do with big storage. The proposition was that seek
> > > > > time is the reason for Tux3's fsync performance. That claim was easily
> > > > > falsified by removing the seek time.
> > > > >
> > > > > Dave's big storage words are there to draw attention away from the fact
> > > > > that XFS ran the Git tests four times slower than Tux3 and three times
> > > > > slower than Ext4. Whatever the big storage excuse is for that, the fact
> > > > > is, XFS obviously sucks at little storage.
> > > > If you allocate spanning the disk from start of life, you're going to
> > > > eat seeks that others don't until later.  That seemed rather obvious and
> > > > straight forward.
> > > It is a logical falacy. It mixes a grain of truth (spreading all over the
> > > disk causes extra seeks) with an obvious falsehood (it is not necessarily
> > > the only possible way to avoid long term fragmentation).
> > You're reading into it what isn't there. Spreading over the disk isn't (just) about avoiding
> > fragmentation - it's about delivering consistent and predictable latency. It is undeniable that if
> > you start by only allocating from the fastest portion of the platter, you are going to see
> > performance slow down over time. If you start by spreading allocations across the entire platter,
> > you make the worst-case and average-case latency equal, which is exactly what a lot of folks are
> > looking for.
> Another fallacy: intentionally running slower than necessary is not necessarily
> the only way to deliver consistent and predictable latency. Not only that, but
> intentionally running slower than necessary does not necessarily guarantee
> performing better than some alternate strategy later.
>
> Anyway, let's not be silly. Everybody in the room who wants Git to run 4 times
> slower with no guarantee of any benefit in the future, please raise your hand.
>
> > > > He flat stated that xfs has passable performance on
> > > > single bit of rust, and openly explained why.  I see no misdirection,
> > > > only some evidence of bad blood between you two.
> > > Raising the spectre of theoretical fragmentation issues when we have not
> > > even begun that work is a straw man and intellectually dishonest. You have
> > > to wonder why he does it. It is destructive to our community image and
> > > harmful to progress.
> > It is a fact of life that when you change one aspect of an intimately interconnected system,
> > something else will change as well. You have naive/nonexistent free space management now; when you
> > design something workable there it is going to impact everything else you've already done. It's an
> > easy bet that the impact will be negative, the only question is to what degree.
> You might lose that bet. For example, suppose we do strictly linear allocation
> each delta, and just leave nice big gaps between the deltas for future
> expansion. Clearly, we run at similar or identical speed to the current naive
> strategy until we must start filling in the gaps, and at that point our layout
> is not any worse than XFS, which started bad and stayed that way.
>
> Now here is where you lose the bet: we already know that linear allocation
> with wrap ends horribly right? However, as above, we start linear, without
> compromise, but because of the gaps we leave, we are able to switch to a
> slower strategy, but not nearly as slow as the ugly tangle we get with
> simple wrap. So impact over the lifetime of the filesystem is positive, not
> negative, and what seemed to be self evident to you turns out to be wrong.
>
> In short, we would rather deliver as much performance as possible, all the
> time. I really don't need to think about it very hard to know that is what I
> want, and what most users want.
>
> I will make you a bet in return: when we get to doing that part properly, the
> quality of the work will be just as high as everything else we have completed
> so far. Why would we suddenly get lazy?
>
> Regards,
>
> Daniel
> --

How?
Maybe this is explained and discussed in a new thread about allocation 
or so.



Thanks
Best Regards
Have fun
C.S.
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