Writing contigiously to UFS2?

Rick C. Petty rick-freebsd at kiwi-computer.com
Wed Sep 26 10:17:58 PDT 2007

On Wed, Sep 26, 2007 at 06:37:18PM +1000, Bruce Evans wrote:
> On Tue, 25 Sep 2007, Rick C. Petty wrote:
> >On Sat, Sep 22, 2007 at 04:10:19AM +1000, Bruce Evans wrote:
> >>
> >>of disk can be mapped.  I get 180MB in practice, with an inode bitmap
> >>size of only 3K, so there is not much to be gained by tuning -i but
> >
> >I disagree.  There is much to be gained by tuning -i: 224.50 MB per CG vs.
> >183.77 MB..  that's a 22% difference.
> That's a 22% reduction in seeks where the cost of seeking every 187MB
> is a few mS every second.  Say the disk speed 61MB/S and the seek cost
> is 15 mS.  Then we waste 15 mS every 3 seconds with 183 MB cg's, or 2%.
> After saving 22%, we waste only 1.8%.

I'm not sure why this discussion has moved into speed/performance
comparisons.  I'm saying 22% difference in CG size.

> Since I
> got to within 1% of the raw disk speed, there is little more to be
> gained in speed here.  (The OP's problem was not speed.)

I agree-- why are you discussing speed?  I mean, it's interesting.  But I
was only discussing CG sizes and suggesting using the inode density option
to reduce the amount of space "wasted" with filesystem metadata.

I do think the performance differences are interesting, but how much of the
differences are irrelevant when looking at modern drives with tagged
queuing, large I/O caches, and reordered block operations?

-- Rick C. Petty

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