03-20-2013
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Mirrors have double write overhead, a minimal issue if there are two cables and controllers involved, but with lots of I/o going on, there it is. On the flip side, your query bandwidth is doubled, as you can read either side. Of course, it is the same controllers, cables, spindles, heads, so total bandwidth for reading is not doubled, just congestion is avoided if a disk might be read in two places with the heads zipping back and forth.
Mirrors can outperform other raids because there is no parity overhead on write, and no parity overhead on a failed read.
If a volume is down, there is no double writing, which helps as there are no longer 2 places to read. When rebuilding the volume, unlike raid it is just a simple copy, but that is when the I/O capabilities are most reduced. While it is a risk, some shops hold off rebuilding volumes until off hours or the weekend.
Mirroring and RAID can happen at different levels of the software and hardware hierarchy. When at a lower level, special hardware can reduce the load. When at a higher level, it is easier to control the cost of activities like rebuilding so there is not a noticable load on the system.
Either way, one critical aspect of these redundant systems is the monitoring, so one failure is ignored until 2 cannot be ignored. Another is competence in repair work! It is tragic to pull the live mirror thinking it is the dead mirror . . . .