AST RESEARCH, INC.        TECHNICAL BULLETIN # 1055                  1-20-95




			   RAID CONSIDERATIONS WITH AST'S MYLEX DAC


PROBLEM
A physical disk drive can not be rebuilt (using hot-swap or hot-standby 
techniques) when using AST's Mylex DAC products.

CAUSE 
When the logical system drive was created, the physical disk drive was not 
supported by the DAC configuration utility software.

PREREQUISITES
Due to the comprehensive nature of the DAC subsystem, AST recommends the 
System Administrator receive proper training prior to configuring this 
subsystem.  This Technical Bulletin assumes the System Administrator has 
already received training from AST and is familiar with the DAC subsystem.

SYSTEMS AFFECTED
AST PART NUMBER           DESCRIPTION
		
501453-001     1-CHANNEL EISA SCSI DISK ARRAY CONTROLLER
501133-001     3-CHANNEL EISA SCSI DISK ARRAY CONTROLLER
501133-002     5-CHANNEL EISA SCSI DISK ARRAY CONTROLLER

PARTS AFFECTED            
AST PART NUMBER         DESCRIPTION

230624-001     CONTROLLER DAC 1-CHANNEL
230414-003     CONTROLLER DAC 3-CHANNEL
230355-XXX     CONTROLLER DAC 5-CHANNEL
221134-001     FRU DAC5 HD CTRL
	
SOLUTION
Reconfigure the logical system drive with the release of DAC utility software 
that supports the physical disk drive.



BACKGROUND
In order for the hot-replacement and hot-standby features to function 
properly, all physical disk drives that are being configured as logical 
drives must be the exact same size.  In most instances, when the same 
manufacturer and model are used, this is not a concern because the drives are 
usually the same size.  It is usually a concern when different manufacturer 
and/or models are used.  AST has qualified 500 megabyte, one gigabyte and two 
gigabyte disk drives for use with the DAC.

AST assigns a single part number for 500 megabyte (230134-500), one gigabyte 
(230135-132 and 230135-133) and two gigabyte (230135-232) SCSI disk drives.  
Several different manufacturers and/or models are sourced under each part 
number.  As a result, (using the one gigabyte disk drive as an example) when 
a one gigabyte disk drive is requested from AST, there is no way to predict 
who the manufacturer and/or model of that disk drive will be when it is 
shipped from AST.  This is a concern because the size of each manufacturer's 
one gigabyte disk drive is not always exactly 1000 megabytes.  In fact, most 
one gigabyte disk drives are slightly larger then 1000 megabytes.  The actual 
size of the disk drive as it is delivered from the manufacturer is called the 
"native" size.  For example, the native size of the Quantum 1080s disk drive 
is 1029 megabytes, and the native size of the HP C2247 disk drive is 1003 
megabytes.  They both need to be exactly 1000 megabytes in order for the 
hot-replacement and hot-standby features to function properly.

INTRODUCING DRIVE SIZING
The native size of each disk drive needs to be modified so that all of the 
disk drives are the same size when they are connected to the DAC.  
This process is called "drive sizing".  In order for the hot-replacement and 
hot-standby features to function properly, all physical disk drives that are 
being configured as logical drives must be the exact same size.  AST provides 
utility software with the DAC to make disk drives with different native sizes 
appear as if they are exactly the same size.  For example, after the Quantum 
1080s disk drive has been sized properly, it is seen by the DAC as 1000 
megabytes instead of 1029 megabytes.

DRIVE SIZING PROCESS
The following dialog reviews the drive sizing process:

AST provides utility software with the DAC to enable system administrators to 
configure one or more independent physical disk drives as a single logical 
disk drive.  A "logical disk drive" is commonly referred to as a "system disk 
drive".  During the course of configuring the system disk drive, each physical 
disk drive provides a drive identification header to the DAC.  
The identification header informs the DAC who the disk drive's manufacturer 
is and the disk drive's model number .  The identification header is cross 
referenced to a file called "ID.DAT".  The ID.DAT file is cross-referenced to 
several files with a file extension called "SIZ".  The file "ID.DAT" also 
contains a listing of all the disk drives that are supported by the DAC.  
Each supported disk drive is listed on a separate line in the file which is 
exactly 16 characters in length, and corresponds to a file with the extension 
"SIZ".  The files with the "SIZ" extension contain the modified size of the 
drive.

DRIVE SIZING EXAMPLE 
The following text resides in the example file /DACCFG/ID.DAT:

	QUANTUM EMPIRE_1
	QUANTUM EMPIRE_5
	HP      C2233-30
	HP      C2235   
	HP      C2244   
	HP      C2247   
	QUANTUM PD1050iS
	QUANTUM LPS525S 
	SEAGATE ST12400N
	SEAGATE ST31200N
	DEC     DSP3210S
	QUANTUM EMPIRE_2
	HP      C3323A

The first line contains the SCSI identification header for the Quantum 1080's 
one gigabyte disk drive.  Because the header is located on the first line, 
it corresponds to the file called "1.SIZ".  The text located in the file 
1.SIZ is "1000".   When the Quantum 1080's disk drive is sized, the DAC does 
not recognize the native size of the disk drive.  Instead, the disk drive is 
recognized as 1000 megabytes.

The sixth line contains the SCSI identification header for Hewlett Packard's 
one gigabyte disk drive.  Because the header is located on the sixth line, 
it corresponds with the file called "6.SIZ".  The text located in the file 
1.SIZ is "1000". When the Hewlett Packard disk drive is sized, the DAC does 
not recognize the native size of the disk drive.  Instead, the disk drive is 
recognized as 1000 megabytes.

After both disk drives have been sized, they are exactly 1000 megabytes and 
can be used in a RAID configuration without any problems.  In this example, 
because two disk drives are being discussed, they can be configured in a 
RAID 1 configuration and the hot-replacement and hot-standby features will 
function properly.

DRIVE SIZING VERIFICATION
It is important for a System Administrator to be able to verify that all of 
the physical disk drives connected to the DAC are properly drive sized.  
Following the procedure below enables the System Administrator to verify the 
disk drives are properly sized:

1.  Boot the latest release of the DAC DOS utility diskette.

2.  Load the mouse driver that accompanies the mouse that is connected to the 
    computer system.

3.  Change to the directory called "DACCFG" by entering the command 
    "CD \DACCFG".

4.  Execute the DAC Disk Array Configuration program by entering the command 
    "DAC960CF".

5.  Click on the "VIEW/UPDATE CONFIGURATION" button.

6.  Click on the "FORMAT SCSI DRIVE" button.  The button is located on the 
    lower right portion of the screen.

7.  Click on the graphical representation of each disk drive.  A box 
    containing several fields will be displayed in the upper left portion of 
    the screen.  Each field reviews specifics about that particular disk 
    drive.  One field needs to be reviewed to verify the disk drive is 
    properly sized.  The first field reads "Capacity [MB] : nnnn", where 
    "nnnn" is the modified size of the drive. For a 500 megabyte disk drive, 
    "nnnn" must be equal to "500".   
    For a one gigabyte disk drive, "nnnn" must be "1000".  For a two gigabyte 
    disk drive, "nnnn" must be equal to "2000". The disk drive is not properly 
    sized if "nnnn" contains any other numbers .

8.  Click on the "EXIT" button that is located in the upper right portion 
    of the screen, next to the clock.

9.  Use the steps outlined above to verify each physical disk drive.

10.  After all disk drives have been reviewed, click on the "QUIT" button 
	to exit the menu.

DRIVE SIZING RESOLUTION
The logical system drive needs to be recreated if one of the physical disk 
drives is not properly sized.  Since recreating a system drive requires 
reinitializing the system drive, all of the data must be backed up prior to 
creating the new system drive.  The data is restored after the system drive 
is initialized.  

Notes
1.  It is important to perform a full system backup prior to initializing a 
    system drive because initializing a system drive will destroy all data.

2.  Please reference the Disk Array User's Manual for the procedure that 
    explains how to use the DAC Configuration Utility to create new system 
    drives.

3.  The latest release of the DAC DOS utility diskette is located on AST's 
    BBS in the Systems file area.  The name of the file is "SMP7.EXE".  AST's 
    BBS can be reached by dialing (817) 230-6850.  AST's BBS supports modem
    speeds up to 28.8k bps.  This file will also be posted on:
	
	- CompuServe 
	   Go ASTSUPPORT

	- AST Worldwide Web Site
	  http://www.ast.com