RE: RAID question

[Neil Palmer <npalmer@xxxxxxxxxxx>]

Err, with all due respect Al !  :-)

Parity is spread over either 4 drives or 8.
You saw the lower available disk capacity of 3145MB on the first 4
drives.
Then you add 2.  You haven't added 4 so the parity information is STILL
only spread over the first 4 drives (your new drives show the full
4194MB available).
If you add another drive it will also show 4194MB available as parity is
still spread over the first 4 drives.
If you add an 8th drive you can now go into DST and RAID-5 will be
spread over all 8 drives, so each will now show 3670MB.
On many controllers you can add up to 10 drives.  So if you added an 8th
or 9th drive the parity information stays spread over the dirst 8
drives, and the 9th & 10th drive will show the full 4194MB available.


Neil Palmer                                AS/400~~~~~      
NxTrend Technology - Canada   ____________          ___  ~     
Thornhill, Ontario,  Canada   |OOOOOOOOOO| ________  o|__||=   
Phone: (905) 731-9000  x238   |__________|_|______|_|______)   
Cell.: (416) 565-1682  x238    oo      oo   oo  oo   OOOo=o\   
Fax:   (905) 731-9202       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
mailto:NPalmer@NxTrend.com    AS/400  The Ultimate Business Server      
http://www.NxTrend.com

> -----Original Message-----
> From: Al Barsa, Jr. [SMTP:barsa2@ibm.net]
> Sent: Friday, September 04, 1998 11:10 AM
> To:   MIDRANGE-L@midrange.com
> Subject:      Re: RAID question
> 
> At 09:12 AM 9/4/98 -0500, you wrote:
> >We just added 2 drives to an S20 that previously had 4 drives on it
> (one
> >RAID set).  I used SST to add the new drives to device parity
> protection
> >and then added them to my ASP.  As we were not adding a full RAID
> set, I
> >expected the system to mirror the devices.  Instead, I see this on
> the
> >WRKDSKSTS display:
> >
> >                    --Protection--            --Protected--
> >Unit      ASP       Type      Status          Size
> >1         1    DPY       ACTIVE          3145
> >2         1    DPY       ACTIVE          3145
> >3         1    DPY       ACTIVE          3145
> >4         1    DPY       ACTIVE          3145
> >5         1    DPY       ACTIVE          4194
> >6         1    DPY       ACTIVE          4194
> >
> >This gives more DASD than if the new drives were irrored, but are
> they
> >really protected?  If so, why am I seeing all 4194 MB?
> >We are at V4R1.
> 
> Good question.  In this case, the system only spread the parity over
> the
> base four drives.  If you turned RAID off and then back again on (in
> which
> case Murphy would have a disk failure at the exact moment of no
> protection), parity would be spread over all six drives, up to a
> maximum of
> 8 on the disk controller on your system with just the base
> controllers.
> 
> Al
> 
> 
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