ZFS committed to the FreeBSD base.

Ulrich Spoerlein uspoerlein at gmail.com
Fri Apr 13 18:55:08 UTC 2007

Rick C. Petty wrote:
> On Thu, Apr 12, 2007 at 01:51:59PM -0500, Craig Boston wrote:
> > For something this low level my opinion is it's better to stay with
> > compile time options.  After all, in the above example, cmpxchg8 is a
> > single machine instruction.  How much overhead does it add to retrieve a
> > variable from memory and check it, then jump to the correct place?
> > Enough that it outweighs the benefit of using that instruction in the
> > first place?
> [...] 
> The problem is that ZFS would be compiled (by default) to work for many
> platforms, and thus a majority of systems wouldn't get the nice
> optimization.

Disclaimer: I have no clue what cmpxchg8 actually does, but ...

We are talking about optimizing a filesystem by speeding up the
necessary CPU computations. Now, whenever the CPU waits for I/O (which
the ZFS threads will do plenty of times) it has literally thousands of
cycles to burn.

I don't see how this could possibly make ZFS any faster if it does not
avoid I/O operations entirely.

Ulrich Spoerlein
"The trouble with the dictionary is you have to know how the word is
spelled before you can look it up to see how it is spelled."
-- Will Cuppy

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