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

From: David Lang
Date: Mon May 11 2015 - 20:13:20 EST

On Mon, 11 May 2015, Daniel Phillips wrote:

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.

This is an interesting idea, but what happens if the files don't arrive as a big batch, but rather trickle in over time (think a logserver that if putting files into a bunch of directories at a fairly modest rate per directory)

And when you then decide that you have to move the directory/file info, doesn't that create a potentially large amount of unexpected IO that could end up interfering with what the user is trying to do?

David Lang
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