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.
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.
--
-- 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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