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Re: software vs. hardware with ide Raid 1



On Mar 26, 2002  14:49 -0600, Steven Lembark wrote:
> -- David Rees <dbr greenhydrant com>
> >On Tue, Mar 26, 2002 at 10:35:01AM -0800, Administrator wrote:
> >>What kind of implementation software or hardware is recommended for ide
> >>Raid 1 with ext3? Some controllers from Promise like their new FastTrak
> >>TX2000 look nice. But won't I be tied into the kernel version that is
> >>only supported under their drivers? Is ide software Raid stable and fast
> >>with ext3 for a production system?
> >
> >I've used both Software RAID1 and the 3ware line of RAID cards with good
> >results.  You can pick up the 6410 (4 port version) for about $100 these
> >days.
> 
> You're nearly always better off using hardware RAID for
> the simple reason that a kernel glitch is that much less
> likely to blow off your storage.

I don't know if I agree with that.  You could just as easily say "you are
better off with software RAID because a hardware glitch is that much less
likely to blow off your storage".  Software RAID has the added benefit
that you are much more likely to be able to fix a software glitch than
you are to fix a hardware problem.

You also have the added benefit with software RAID that you are not stuck
with a particular vendor's striping/layout/metadata/etc in case your
card dies.  With software RAID you just need a JBOD setup (which can
be done with most cards, and can even be done in a pinch with multiple
adapter cards).

You are also isolated from situations where the card firmware/software
decides you need to reinitialize your array.  At least with software
RAID you can hack the data on disk (because it is a well-known layout)
to override whatever you really need.

Next, you are much better able to do RAID over multiple adapters with
software instead of with hardware RAID (unless you are talking about
very expensive external RAID enclosures).

Finally, there is really a performance tradeoff between hardware and
software RAID.  In one sense, hardware RAID (esp. RAID 5) can offload the
checksums from the CPU, and only passes the data over the bus a single
time (for RAID 1) to improve performance.  This is offset by the fact
that it is usually much cheaper to upgrade your CPU and RAM than it is
to improve the performance of a hardware RAID setup.

Cheers, Andreas
--
Andreas Dilger  \ "If a man ate a pound of pasta and a pound of antipasto,
                 \  would they cancel out, leaving him still hungry?"
http://www-mddsp.enel.ucalgary.ca/People/adilger/               -- Dogbert





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