[Ham-Computers] RE: To RAID or not to RAID, that is the question

[Hsu, Aaron (NBC Universal)]

>>> You wrote: 
Aaron didn't comment on your statement about a system being limited to 4
EIDE drives.  You don't have any apparent need to go in this direction
but the number of EIDE drives, like the number of SCSI or SATA drives is
limited by the number and type of controllers.  Our main box here has 6
EIDE (two pair of RAID 1 and two standalone) and a CDRW and a DVDRW
drive.  

>>> My reply:
I think I understand what you meant to say, but it didn't quite come out
that way and may have confused some people as your 6-drive setup breaks
the 4-drive limitation you stated.

In reality, current "systems" have no limits as to the number of drives
(even EIDE) that can be installed.  The only limits are due to port
availability and/or OS support.  If we include "legacy" limitations,
then yes, there is a 4-drive limit due to the number of "standard" I/O
ports assigned to "hard drive" controllers (starting at 0x1F0 and
0x170).  Each controller could have a max of two drives and with two
controllers, a max of 4 "standard" hard drives were supported on a
system (via the legacy ST-506 access methods).  On legacy systems,
controllers that used "non-standard" ports, such as SCSI, could support
additional drives (up to 15 on a "wide" SCSI bus) and operating systems
would recognize the drives as long as proper drivers were installed or
the controller BIOS hooked into INT13.

These days, all systems and OS' have support for more than 4 drives.  If
a controller doesn't use the standard "legacy" ports, then the drives
may not show up in the OS (or POST) until drivers are installed (unless
the OS has native support to detect mass storage devices).  You can
easily add additional EIDE drives by adding additional EIDE controllers,
as long as these controllers can be configured to use ports other than
0x1F0 and 0x170 that the primary and secondary controllers use.

73!

  - Aaron Hsu, NN6O