Path: utzoo!attcan!utgpu!news-server.csri.toronto.edu!rutgers!apple!agate!eos!amelia!sun418!truesdel From: truesdel@sun418.nas.nasa.gov (David A. Truesdell) Newsgroups: comp.unix.i386 Subject: Re: Disk Mirroring (was Re: Altos 5000) Message-ID: Date: 31 Aug 90 06:16:42 GMT References: <1990Aug16.174514.2646@NCoast.ORG> <15759@bfmny0.BFM.COM> <1990Aug27.183821.13518@ico.isc.com> <3895@altos86.Altos.COM> Sender: news@amelia.nas.nasa.gov Lines: 39 dtynan@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@ico.isc.com>, rcd@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@prandtl.nas.nasa.gov)