On Tue, Dec 11, 2001 at 11:06:33AM +1030, Greg Lehey wrote:
> On Monday, 10 December 2001 at 10:30:04 -0800, Matthew Dillon wrote:
> >
> >>> performance without it - for reading OR writing. It doesn't matter
> >>> so much for RAID{1,10}, but it matters a whole lot for something like
> >>> RAID-5 where the difference between a spindle-synced read or write
> >>> and a non-spindle-synched read or write can be upwards of 35%.
> >>
> >> If you have RAID5 with I/O sizes that result in full-stripe operations.
> >
> > Well, 'more then one disk' operations anyway, for random-I/O. Caching
> > takes care of sequential I/O reasonably well but random-I/O goes down
> > the drain for writes if you aren't spindle synced, no matter what
> > the stripe size,
>
> Can you explain this? I don't see it. In FreeBSD, just about all I/O
> goes to buffer cache.
>
> > and will go down the drain for reads if you cross a stripe -
> > something that is quite common I think.
>
> I think this is what Mike was referring to when talking about parity
> calculation. In any case, going across a stripe boundary is not a
> good idea, though of course it can't be avoided. That's one of the
> arguments for large stripes.
In a former life I was involved with a HB striping product for SysVr2
that had a slightly modified filesystem that 'knew' when it was
working on a striped disk. And as it know, it avoided posting I/O s
that crossed stripes.
W/
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