Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!mips!zaphod.mps.ohio-state.edu!usc!snorkelwacker!paperboy!meissner From: meissner@osf.org (Michael Meissner) Newsgroups: comp.unix.i386 Subject: Re: Disk Mirroring (was Re: Altos 5000) Message-ID: Date: 31 Aug 90 14:04:20 GMT References: <1990Aug16.174514.2646@NCoast.ORG> <15759@bfmny0.BFM.COM> <1990Aug27.183821.13518@ico.isc.com> <3895@altos86.Altos.COM> Sender: news@OSF.ORG Followup-To: comp.unix.i386 Organization: Open Software Foundation Lines: 34 In-reply-to: dtynan@altos86.Altos.COM's message of 31 Aug 90 02:12:51 GMT In article <3895@altos86.Altos.COM> dtynan@altos86.Altos.COM (Dermot Tynan) writes: | See above. Nobody is trying to produce a fault-free system. We are just | trying to reduce the likelihood of having to restore a filesystem. Believe | me. Disk mirroring will slow down disk writes (which aren't the bulk of | disk operations, anyway), but it will double your disk reliability. If both mirrors are operational, it can speed up reads, since the system will get the data from which ever disk's read head is closer (assuming a smart OS and/or controller). Another win with disk mirroring is the trick they used internally on at least one machine at Data General. The main OS machine had a disk farm that was getting to the point that backups could no longer be done in a reasonable time period. What they did was mirror some/all of their critcal drives. Then they would break the mirror, and start backups on one side of the mirror (they could break the mirror without any disruption or taking the disk offline). Meanwhile, the users would be busily writing to the other (now non-mirrored) disk. This way backups did not have data changing underneath, they could use the much faster raw disk backup procedure (dump instead of tar in UNIX-speak), and the system did not have to be taken down. When the backups finished, they regrafted the mirrored disks back together, and the system would resync the disks during the idle loop. The downside of any mirroring scheme of course, is that you have to buy twice as many disk drives as you did previously (and I never was in a group that could afford it :-). -- Michael Meissner email: meissner@osf.org phone: 617-621-8861 Open Software Foundation, 11 Cambridge Center, Cambridge, MA, 02142 Do apple growers tell their kids money doesn't grow on bushes?