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I found a list of versions and their Solaris release numbers

I know that you can grow a pool by replacing drives with larger ones or adding new drives or mirrors to the pool. I heard that ZFS did not yet support shrinking pools by removing drives/mirrors. But that has probably been changed.

Which version (if any) released the ability to shrink a pool?

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A really desired feature if you have disk fail with no ability to find a replacement. In that case some free space can be traded-in to restore the ZRAID redundancy. – Aleksandr Levchuk Apr 12 '11 at 23:15
@Aleksandr: Interesting concept :) You could use free space for redundancy even if there was no failure in disks. You would just have to make sure that the feature will trade in redundancy for extra space if you have too much usage, or if the usage goes up right after a disk failure. (better cap your usage or else it is possible to lose redundancy even without disk failure) (I am glad I am not the programmer making it work!) – George Bailey Apr 12 '11 at 23:28
up vote 7 down vote accepted

No current release can shrink a pool and I have not heard any announcements of an upcoming feature to do so. Considering how rare it is that a properly designed pool would need the ability, I wouldn't expect it anytime soon.

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As @jan pointed out, there some feature in the works, that does at least allow some forms of vdev removal. Nothing released as of yet, but at least an announcement – Slizzered Apr 10 '15 at 11:15

I cant find that it is supported, yet anyways.

Here's a printout from my Solaris 11 Express running ZFS pool version 31.

zpool upgrade -v
This system is currently running ZFS pool version 31.

The following versions are supported:

---  --------------------------------------------------------
 1   Initial ZFS version
 2   Ditto blocks (replicated metadata)    
 3   Hot spares and double parity RAID-Z    
 4   zpool history    
 5   Compression using the gzip algorithm         
 6   bootfs pool property    
 7   Separate intent log devices    
 8   Delegated administration    
 9   refquota and refreservation properties    
 10  Cache devices    
 11  Improved scrub performance    
 12  Snapshot properties    
 13  snapused property    
 14  passthrough-x aclinherit    
 15  user/group space accounting    
 16  stmf property support    
 17  Triple-parity RAID-Z    
 18  Snapshot user holds    
 19  Log device removal    
 20  Compression using zle (zero-length encoding)    
 21  Deduplication    
 22  Received properties    
 23  Slim ZIL    
 24  System attributes    
 25  Improved scrub stats    
 26  Improved snapshot deletion performance    
 27  Improved snapshot creation performance    
 28  Multiple vdev replacements    
 29  RAID-Z/mirror hybrid allocator   
 30  Encryption    
 31  Improved 'zfs list' performance

For more information on a particular version, including supported releases,
see the ZFS Administration Guide.
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It looks that Alex Reece works on implementing the feature in the OpenZFS project: OpenZFS Device Removal blog.

Thorough ZFS in Solaris and OpenZFS are two different projects (see Wikipedia: ZFS).

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