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I'm not able to find a clear yes/no answer to this simple question: does using ZFS in mirror mode double the read performance of the underlying vdevs?

This applies to either individual requests or net performance (i.e. a single read request is sharded across two disks or two simultaneous requests are each served simultaneously from different disks).

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  • why you don't use iostat to see your disk activity in the mid time your are reading from your raid? i did the same with lvm mirror in the read activity was on one of two disk, i don't see the sense to read the data twice
    – c4f4t0r
    Commented Mar 13, 2015 at 17:22
  • 1
    This would, at the very least, depend on the implementation: ZFS doesn't do anything with reads, because it's just a file system. Hence, specify the implementation you are using and this might be answerable.
    – user
    Commented Mar 14, 2015 at 20:35
  • ZFS is not "just a file system". It is a filesystem and a disk subsystem - look into vdevs. Commented Mar 20, 2015 at 17:21

2 Answers 2

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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)

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  • 6
    As of early 2016 ZFS on Linux uses the same FreeBSD patch, which incidentally replaced an earlier patch from 2013 that already improved on round-robin.
    – Bob
    Commented Jul 3, 2017 at 8:14
  • Good points, this answer is really outdated now. ZFS seems to be much more synchronized these days so I'm not sure if it really matters anymore.
    – feitingen
    Commented Feb 13, 2018 at 14:08
  • What about the state of this situation in the year 2020?
    – Akito
    Commented Feb 9, 2020 at 13:54
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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.

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  • Where are you seeing an increase of 50%? The author reports 204 MB/s for a single 4 TB HDD and 488 MB/s for a 2x mirror.
    – Bob
    Commented Jun 17, 2023 at 22:29

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