Does ZFS in mirror mode double vdev read performance?

Solution 1:

If using Linux/FreeBSD (also FreeNAS) read speed is much better

Source: http://open-zfs.org/wiki/Features#Improve_N-way_mirror_read_performance It's hard to tell whether illumos/OSX have that patch which rebalances read I/O to the least busy device, or just improved round-robin.

In any case, mirror improves read I/O. As the other answer mentions, using SSDs usually doubles read I/O, while spinning drives increase performance by less than double depending on type of load.

My anecdotal testing on Linux shows a significant speed increase, especially when reading large files or more at the same time.

(Edited to reflect changes since the answer was posted)

Solution 2:

This depends on the ZFS implementation and the Hardware you are using.

These benchmarks https://calomel.org/zfs_raid_speed_capacity.html provide a quick overview of the mirrored/striped performance, measured on FreeBSD.

As you can see, the read performance is doubled with SSDs and increased by approximately 50% with HDDs.