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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).

  • 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 Mar 13 '15 at 17:22
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    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. – a CVn Mar 14 '15 at 20:35
  • ZFS is not "just a file system". It is a filesystem and a disk subsystem - look into vdevs. – Mahmoud Al-Qudsi Mar 20 '15 at 17:21
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If using FreeBSD (also FreeNAS) read speed is better.

Source: http://open-zfs.org/wiki/Features#Improve_N-way_mirror_read_performance

Linux/illumos/OSX does not have that patch which rebalances read I/O to the least busy device, instead it's just round-robin.

With these, you will still get a performance boost with a n-way mirror vs. single drive, but in most usage scenarios, performance won't be doubled since mixed large/small requests aren't balanced and thus the read I/O to each leg of the mirror won't be balanced.

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    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 Jul 3 '17 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  • Good points, this answer is really outdated now.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 Feb 13 '18 at 14:08
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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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