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Re: hardware raid vs. software raid

On Fri, Dec 01, 2006 at 09:19:08AM +0100, Matus UHLAR - fantomas wrote:
> On 30.11.06 17:00, Roberto C. Sanchez wrote:
> > This is only true if you use IDE and also put the mirrored pair on the
> > same channel (which would be incredibly foolish).  Under every other
> > conceivable scenario, the system is smart enough to do both writes
> > simultaneously since they will be going difference places.
> but the data will still be fetched two times from memory to a controller(s)

I don't understand why this matters.  The miracle of DMA makes it so
that this happens without CPU intervention.  The fact that you can put
the drives on separate channels makes it so that the operations can
happen in parallel.  In practice, RAID-1 performance is the same as
non-RAID for writing and up to twice as fast for reading.

> > > - software RAID-1 has high CPU and bus overhead in case of resyncing after
> > > disk outage (hw raid does that itself)
> > 
> > Huh?  I have built lots of RAID one systems and seen many of them
> > resync.  They do have high I/O overhead (naturally, since it is a
> > resync).  However, I have never seen one produce high CPU overload.
> I should write "higher CPU overload"
I should have written "high CPU load."  Basically, the CPU only gets
loaded if you have DMA disabled.

> > I suppose if you forgot to enable DMA on your drives, or somehow managed
> > to disable it (it is enabled by default), then you might see high CPU load
> > during resync.  But then, you would see high CPU load for anything that
> > does any significant amount of I/O.



Roberto C. Sanchez

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