Message-ID: <554272F1.80801@ontolab.com>
Date:	Thu, 30 Apr 2015 20:22:41 +0200
From:	Christian Stroetmann <stroetmann@...olab.com>
To:	Daniel Phillips <daniel@...nq.net>
CC:	Howard Chu <hyc@...as.com>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Dave Chinner <david@...morbit.com>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	tux3@...3.org, Theodore Ts'o <tytso@....edu>,
	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
Subject: Re: xfs: does mkfs.xfs require fancy switches to get decent performance?
 (was Tux3 Report: How fast can we fsync?)

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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