Disk Mirroring (was Re: Altos 5000)

[David A. Truesdell]

dtynan at altos86.Altos.COM (Dermot Tynan) writes:

[ Quite a bit about mirrored filesystems, which I won't repeat here. ]

Disk mirroring IS a relatively inexpensive method of "hardening" modest amounts
of data.  However, when you want to protect more than just a few disks worth,
the costs of buying duplicate drives can quickly get out of hand.

A less expensive approach, if you have a LOT of data, is to use a RAID
(Redundant Array of Inexpensive Disks) style system, which can use a single
spare disk to protect the data on several others.  When you are talking about
100's of gigabytes of data, that's a lot of disk drives you won't have to buy.
(Purists may note that mirroring is considered a simple form of RAID.)

>In article <1990Aug27.183821.13518 at ico.isc.com>, rcd at ico.isc.com (Dick Dunn) writes:
>> I had pointed out that it takes extra I/O bandwidth to handle mirroring;
>> someone responded that if you have the right sort of controller, it will
>> write both disks at once for you.  OK, fine, now you've made the controller
>> a single-point-of-failure.

>	MTBF(controller) >> MTBF(disks)		Get it?

>> I've seen as many motherboard and controller
>> failures as disk failures.  I don't pretend my experience is typical, but
>> suppose that it might be.  The disks are not the only failure points in the
>> system.

>I suggest that you have some serious design flaws here.

Another design flaw would be to use a single controller to run both disks.
Separate controllers running, running separate disks, could allow the system
to continue running in spite of the failure of a controller or a disk.  If you
get the software right, you would only have to come down long enough to replace
a controller.  (If you get the hardware right, wouldn't have to do that!)

--

T.T.F.N.,
dave truesdell (truesdel at prandtl.nas.nasa.gov)