Re: RAID question

Jeff,

  In this case, You WIN!

  Any raid set starts with either 4 (your case) or 8 drives.  The
information needed to recover any ONE failed drive in the set is spread
across the initial drives thus the 4GB units you have report 3GB usable
(1GB each is used for the raid recovery information)  If the set started
at 8 units then the drives would report about 3.5 Usable each.  You can
see that in any set you lose ONE disk of capacity no matter how you
slice it.  The AS/400 can utilize up to 10 drives in a set in either
case.  So when you added the two new drives they were added to the set
you already have.  Since the required RAID space has been reserved on
the first 4 units, the remaining units get to utilize their full size.  
The primary benefit of starting with 8 units instead of four is when a
unit fails, the information needed to 'fake' the failed unit comes from
7 or 8 rather than 3 or 4 units (depending on if the failed unit is one
of the drives with the raid information on it or not).

  hth

  Larry Bolhuis
  Arbor Solutions, Inc
  lbolhui@ibm.net
  

Jeff Carey 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.
> 
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