On Mon, 16 Jul 2012, Stefan Ring wrote:

It is normal for reads from mirrors to be faster than for a single disk
because reads can be scheduled from either disk, with different I/Os being
handled in parallel.

That assumes that there *are* outstanding requests to be scheduled in
parallel, which would only happen with multiple readers or a large
read-ahead buffer.

That is true. Zfs tries to detect the case of sequential reads and requests to read more data than the application has already requested. In this case the data may be prefetched from the other disk before the application has requested it.

Bob Friesenhahn
bfrie...@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/
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