> > > I discovered the disk drives can be on opposite sides of the world if you > > > like, so long as you have the network capacity.
> > With Gigabit Ethernet (and faster) becoming more widely available, this kind of > > facility will become more feasible. > Only for limited uses. Latency is the chief limiter for many applications > and the speed of light actually gets to be annoyingly slow IBM techies discussed some interesting ideas recently. One idea is just to transmit changes. If you have mirrored disks that are geographically separated, you can look at the old record on the driving system before you replace it with the new. XORing a changed 4KB block with the original usually produces just a few dozen bytes of changes - with a compressing tranmission channel the binary zeroes (where the data was unchanged) all compress away. Transmit the result of the XOR and XOR it with the remote copy to recreate the changed block. Initial tests suggest an eightfold circuit capacity reduction is possible. Doesn't solve the latency problem, but cuts the bandwidth requirement. -- Phil Payne http://www.isham-research.com +44 7785 302 803 +49 173 6242039
