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Re: [K12OSN] Need Help



Several good points, Les.  Responses inline.

--TP

Les Mikesell wrote:
> Write access is considerably slower on RAID5 and it tends to lock your
> heads together even for reads.  I've always liked RAID1 for the simple
> reason that if everything is broken except one disk you can still
> recover the data it held.  Plus if you do it in software you don't
> have to worry about having to match the controller to read on a
> different machine.
However, RAID 1, by definition, is not scalable beyond two disks.  So
you're in a very similar situation (losing one disk and still going)
here as you are with RAID 5.  And as for RAID 5 locking the heads
together even for reads, that may well depend on your specific RAID
card.  We haven't seen any evidence of that with our systems at work. 
However, it might well be true in some implementations, maybe to include
software RAID.

>> That said, RAID 5 kicks RAID 1 in the delicate parts when it
>> comes to performance.  Again, we're back to, say, six or eight spindles
>> vs. two spindles; no contest.
>
> That's not necessarily true.  If you configured those 8 drives in
> RAID1 pairs, you'd have 4 independently seeking places that could be
> writing at once and all 8 would be independent for reads.  The trick
> is to arrange your data across the partitions so they are likely to be
> used simultaneously.  These days you could just combine the RAID1 sets
> into one LVM, though.
>
You're no longer talking about RAID 1, though.  You're talking about
RAID 10.

> > I've run many 14-disk SCSI RAID 5 setups,
>> and my God, they were quick!!  Yes, I'm assuming a real hardware RAID
>> card here; I generally don't recommend software RAID, no matter which
>> RAID level you use.
>
> Software RAID1 works very nicely and does not add much overhead on
> SCSI where there is not much CPU interaction anyway.  I probably
> wouldn't do RAID5 in software.
>
Largely true with SCSI; I wasn't specific enough in that second
sentence.  Oh, how I wish all disk drive interfaces were SCSI--that
would solve plenty of problems!  But even with SCSI, we're back to the
scalability issue; RAID 1 cannot, by definition, scale beyond two
spindles, so if you want larger capacity, you've either got to buy two
*huge* drives or go with RAID 5.  I certainly agree with you about not
doing RAID 5 in software; that calls for a hardware controller.  And if
you do go with hardware RAID controllers, then your CPU usage becomes
independent of whether you're using PATA, SATA, or SCSI, since the RAID
card does all that work.


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