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

From: Daniel Phillips
Date: Thu Apr 30 2015 - 11:14:21 EST

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?


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