Re: Reverse of XOR.

Hello, Walden:

Actually, the XOR technique you illustrate is exactly how the OLD
"Check-Sum" protection for disk drives worked on the System/38,
and the early AS/400s, prior to RAID-5.  If you go back and look
at the documentation for "check-sum" DASD, you will see that,
for every two disk drives you wanted to protect, you needed to
add a third disk drive (to contain the XOR data of the first two).

RAID-5 is based on slightly different technology, using "striping"
and overlapping parity, etc., to acheive the same effects, similar
to ECC for main memory.

A quick search at www.google.com reveals:
==============================================
Short for Redundant Array of Independent (or Inexpensive) Disks, a category
of disk drives that employ two or more drives in combination for fault
tolerance and performance. RAID disk drives are used frequently on servers
but aren't generally necessary for personal computers.
There are number of different RAID levels. The three most common are 0, 3,
and 5:

Level 0: Provides data striping (spreading out blocks of each file across
multiple disks) but no redundancy. This improves performance but does not
deliver fault tolerance.
Level 1: Provides disk mirroring.
Level 3: Same as Level 0, but also reserves one dedicated disk for error
correction data. It provides good performance and some level of fault
tolerance.
Level 5: Provides data striping at the byte level and also stripe error
correction information. This results in excellent performance and good fault
tolerance.
==============================================
RAID-5. This type includes a rotating parity array, thus addressing the
write limitation in RAID-4. Thus, all read and write operations can be
overlapped. RAID-5 stores parity information but not redundant data (but
parity information can be used to reconstruct data). RAID-5 requires at
least three and usually five disks for the array. It's best for multi-user
systems in which performance is not critical or which do few write operation
s.
==============================================
See this URL for more of the details:
http://searchstorage.techtarget.com/sDefinition/0,,sid5_gci214332,00.html

So, conceptually, the XOR idea is what led to the evolution of what
eventually became RAID, which uses "striping" and other techniques
of "overlapping parity", essentially similar to the extra memory bits
for ECC (Error Correcting Codes) used in the main storage (RAM)
of the iSeries and mainframes.

With such a scheme of overlapping parity bits, for example, to
protect a 32-bit fullword,  ECC adds 5 extra bits, and detects all
two bit errors and corrects all 1-bit errors within the 32-bits of data.
(This is off the top of my head; I think the ratio of 5 bits to protect
32 bits is correct, but I might be mistaken.)

Regards,

Mark S. Waterbury

----- Original Message -----
From: "Walden H. Leverich" <WaldenL@TechSoftInc.com>
To: <mi400@midrange.com>
Sent: Tuesday, August 27, 2002 9:31 AM
Subject: RE: [MI400] Reverse of XOR.


> >From: James H. H. Lampert [mailto:jamesl@hb.quik.com]
> >This, by the way, is why XOR operations are often used for graphics
>
> And more relevant to the iSeries, XOR is the basis for RAID-5 disk
> subsystems. Given three drives:
>
> Bit Drive1 Drive2 Drive3
> 1 1 1 0
> 2 1 0 1
> 3 0 0 1
> 4 0 1 1
>
> Etc. Remove (crash) any one of those drives and you can recover the data
on
> it by XOR-ing the data on the other two drives.
>
> -Walden
>
>
> ------------
> Walden H Leverich III
> President
> Tech Software
> (516) 627-3800 x11
> (208) 692-3308 eFax
> WaldenL@TechSoftInc.com
> http://www.TechSoftInc.com
>
> Quiquid latine dictum sit altum viditur.
> (Whatever is said in Latin seems profound.)