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

[Daniel Phillips]

Hi Pavel,

On 05/11/2015 03:12 PM, Pavel Machek wrote:
>>> 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.
> 
> Umm, are you sure. If "some areas of disk are faster than others" is
> still true on todays harddrives, the gaps will decrease the
> performance (as you'll "use up" the fast areas more quickly).

That's why I hedged my claim with "similar or identical". The
difference in media speed seems to be a relatively small effect
compared to extra seeks. It seems that XFS puts big spaces between
new directories, and suffers a lot of extra seeks because of it.
I propose to batch new directories together initially, then change
the allocation goal to a new, relatively empty area if a big batch
of files lands on a directory in a crowded region. The "big" gaps
would be on the order of delta size, so not really very big.

Anyway, some people seem to have pounced on the words "naive" and
"linear allocation" and jumped to the conclusion that our whole
strategy is naive. Far from it. We don't just throw files randomly
at the disk. We sort and partition files and metadata, and we
carefully arrange the order of our allocation operations so that
linear allocation produces a nice layout for both read and write.

This turned out to be so much better than fiddling with the goal
of individual allocations that we concluded we would get best
results by sticking with linear allocation, but improve our sort
step. The new plan is to partition updates into batches according
to some affinity metrics, and set the linear allocation goal per
batch. So for example, big files and append-type files can get
special treatment in separate batches, while files that seem to
be related because of having the same directory parent and being
written in the same delta will continue to be streamed out using
"naive" linear allocation, which is not necessarily as naive as
one might think.

It will take time and a lot of performance testing to get this
right, but nobody should get the idea that it is any inherent
design limitation. The opposite is true: we have no restrictions
at all in media layout.

Compared to Ext4, we do need to address the issue that data moves
around when updated. This can cause rapid fragmentation. Btrfs has
shown issues with that for big, randomly updated files. We want to
fix it without falling back on update-in-place as Btrfs does.

Actually, Tux3 already has update-in-place, and unlike Btrfs, we
can switch to it for non-empty files. But we think that perfect data
isolation per delta is something worth fighting for, and we would
rather not force users to fiddle around with mode settings just to
make something work as well as it already does on Ext4. We will
tackle this issue by partitioning as above, and use a dedicated
allocation strategy for such files, which are easy to detect.

Metadata moving around per update does not seem to be a problem
because it is all single blocks that need very little slack space
to stay close to home.

> Anyway... you have brand new filesystem. Of course it should be
> faster/better/nicer than the existing filesystems. So don't be too
> harsh with XFS people.

They have done a lot of good work, but they still have a long way
to go. I don't see any shame in that.

Regards,

Daniel