Subject: Compaq Systempro dataguarding
Date: 4/6/90

Question:

Does anyone know the specific differences between data-guarding,
mirroring & duplexing on the Compaq SystemPro?  Presumably you do
not need NetWare for data-guarding or mirroring, and
data-guarding will result in performance degradation if a drive
fails, where mirroring will not.  SFT NetWare is required for
duplexing.

But how does data-guarding work, and how does it compare
specifically with mirroring?  How does Compaq's hardware
mirroring compare with SFT NetWare mirroring?


Answer:

Just to clear up a miss understanding on the DataGuarding feature
on the Compaq IDA.  DataGuarding is NOT a data compression!
DataGuarding is an exclusive or (XOR) of drives 1,2, and 3.  The
result is store on the fourth drive.  This allow the driver that
resides on the IDA controller to build the data on the fly if one
of the drives were to fail.  The data might look like this:

Drive 1         Drive 2         Drive 3         Drive 4
-------         -------         -------         -------
  0               0               0               0
  0               0               1               1
  0               1               0               1
  0               1               1               0
  1               0               0               1
  1               0               1               0
  1               1               0               0
  1               1               1               1


If drive two were to fail you would have enough information to
rebuild the data that existed on the drive.  The controller would
perform and XOR on the other three drives.

Drive 1         Drive 2         Drive 3         Drive 4  Rebuild
-------         -------         -------         -------  -------
  0               X               0               0	       0
  0               X               1               1	       0
  0               X               0               1	       1
  0               X               1               0        1
  1               X               0               1        0
  1               X               1               0        0
  1               X               0               0        1
  1               X               1               1        1

The nice thing about the data guard is that you protect yourself
against the lost of a drive without having to give up 50% of your
data storage potential.  In other words, a level of protection
between disk mirroring and non disk mirrored systems.