RE: RAID question

>From the AS/400e series System Handbook

(there are many RAID-5 controllers but the rules are the same)

PCI RAID Disk Unit Controller Ultra SCSI #2726
The PCI RAID Disk Unit Controller #2726 supports a maximum of 15 one or
two-byte disk units.
A minimum of four disk units of equal capacity are required to implement
RAID-5 protection.
A maximum of 10 disk units per RAID-5 array are supported.
Parity information can be spread across four or eight disks.
A maximum of three RAID-5 arrays are supported on one #2726.

PCI RAID Disk Unit Controller Ultra SCSI #2740
The #2740 is functionally equivalent to the #2726 PCI RAID Disk Unit
Controller.
However, the #2740 can only be located in the System Unit.

PCI RAID Disk Unit Controller Ultra SCSI #2741
The #2741 is functionally equivalent to the #2726 PCI RAID Disk Unit
Controller.
However, #2741 offers an enhancement over the #2726 in that it will
support compression.



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


> -----Original Message-----
> From: Al Barsa, Jr. [SMTP:barsa2@ibm.net]
> Sent: Friday, September 04, 1998 1:47 PM
> To:   MIDRANGE-L@midrange.com
> Subject:      RE: RAID question
> 
> At 11:03 AM 9/4/98 -0600, you wrote:
> 
> I respectfully disagree.  I originally purchased an S10 with 5 4 GB
> drives.
>  The parity was equally spread over all 5 drives.  I then purchased
> another
> 5 4GB drives, and it set up another parity set!  (So I was now wasting
> two
> full drives for redundancy.)  When I meekly and mildly expressed my
> disappointment to Rochester (No, not the man that used to polish Mr.
> Benny's car.) I was informed that if I added less than four drives at
> a
> time, they would have been added to the same parity set, but four or
> more
> went into their own parity set.  To recoup the lost space, I had to
> end
> RAID and restart it.  The system then put the parity on the first
> eight
> drives, so I only lost one for redundancy.
> 
> Al
> 
> 
> >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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