Disk Mirroring (was Re: Altos 5000)

Michael Meissner meissner at osf.org
Sat Sep 1 00:04:20 AEST 1990

In article <3895 at altos86.Altos.COM> dtynan at 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 at 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?

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