Date:	Thu, 30 Apr 2015 17:00:42 +0100
From:	Howard Chu <hyc@...as.com>
To:	Daniel Phillips <daniel@...nq.net>,
	Mike Galbraith <umgwanakikbuti@...il.com>
Cc:	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?)

Daniel Phillips wrote:
> On 04/30/2015 07:28 AM, Howard Chu wrote:
>> 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.

Totally agree with you there.

> Not only that, but
> intentionally running slower than necessary does not necessarily guarantee
> performing better than some alternate strategy later.

True, it's a question of algorithmic efficiency - does the performance 
decay linearly or logarithmically.

> 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.

git is an important workload for us as developers, but I don't think 
that's the only workload that's important for us.

>>>> 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?

I never said anything about getting lazy. You're working in a closed 
system though. If you run today's version on a system, and then you run 
your future version on that same hardware, you're doing more CPU work 
and probably more I/O work to do the more complex space management. It's 
not quite zero-sum but close enough, when you're talking about highly 
optimized designs.

-- 
   -- Howard Chu
   CTO, Symas Corp.           http://www.symas.com
   Director, Highland Sun     http://highlandsun.com/hyc/
   Chief Architect, OpenLDAP  http://www.openldap.org/project/
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