Here is one example how to import zfs pools from previous RHEL6 to RHEL7

On the old RHEL6 system, or even a RHEL7 server, export the ZFS pools first.

Export ZFS pools

Though It's not big deal if you don't export ZFS pools. It's better to do so.

ZFS export command exports the given pools from the system. All devices are marked as exported, but are still considered in use by other subsystems. Then the devices can be moved between systems and imported as ong as a sufficient number of devices are present.

run

zpool export <pool>
or
zpool export -a # exports all pools imported on the system.
The '-f' option also can be used to forcefully unmount all datasets, using "umount -f" command. However this may lead to potential data corruption.

Note:   Before exporting the pool, all datasets within the pool are unmounted. A pool can not be exported if it has a shared spare  that  is  currently being used.

Import ZFS pools

On the other machine, or the same machine after reinstallation( for example reinstalled RHEL7 from RHEL6)

ZFS module

Load the module first, run

[root@fibrevillage02 ~]# modprobe zfs

Check available ZFS pools can be imported

run command see how many ZFS pools can be imported.

[root@fibrevillage02 ~]# zpool import
   pool: fibrevillage02_1
     id: 1778952909076962034
  state: ONLINE
 status: Some supported features are not enabled on the pool.
 action: The pool can be imported using its name or numeric identifier, though
    some features will not be available without an explicit 'zpool upgrade'.
 config:

    fibrevillage02_1                                                   ONLINE
      raidz2-0                                                  ONLINE
        ata-HITACHI_H7220AA30SUN2.0T_0949M5EVJT_JK1130YAH5EVJT  ONLINE
        ata-HITACHI_H7220AA30SUN2.0T_0949M5BXKT_JK1130YAH5BXKT  ONLINE
        ata-HITACHI_H7220AA30SUN2.0T_0949M570WT_JK1130YAH570WT  ONLINE
        ata-HITACHI_H7220AA30SUN2.0T_0949M4Z9JT_JK1130YAH4Z9JT  ONLINE
        ata-HITACHI_H7220AA30SUN2.0T_0949M56NTT_JK1130YAH56NTT  ONLINE
        ata-HITACHI_H7220AA30SUN2.0T_0949M56MTT_JK1130YAH56MTT  ONLINE
        ata-HITACHI_H7220AA30SUN2.0T_0949MYJ48T_JK1130YAGYJ48T  ONLINE
        ata-HITACHI_H7220AA30SUN2.0T_0949M570RT_JK1130YAH570RT  ONLINE
        ata-HITACHI_H7220AA30SUN2.0T_0946MYG7ET_JK1130YAGYG7ET  ONLINE
        ata-HITACHI_H7220AA30SUN2.0T_0949M56V6T_JK1130YAH56V6T  ONLINE
        ata-HITACHI_H7220AA30SUN2.0T_0949M4ZNBT_JK1130YAH4ZNBT  ONLINE
...

    fibrevillage02_4                                                   ONLINE
      raidz2-0                                                  ONLINE
        ata-HITACHI_H7220AA30SUN2.0T_0949M4WYPT_JK1130YAH4WYPT  ONLINE
    ...

In the example above, we found 4 ZFS pools.

Import  ZFS pools

Import the old pool by running the command below, and check the status

[root@fibrevillage02 ~]# zpool import fibrevillage02_1
[root@fibrevillage02 ~]# zpool status
  pool: fibrevillage02_1
 state: ONLINE
status: Some supported features are not enabled on the pool. The pool can
    still be used, but some features are unavailable.
action: Enable all features using 'zpool upgrade'. Once this is done,
    the pool may no longer be accessible by software that does not support
    the features. See zpool-features(5) for details.
  scan: scrub repaired 0 in 22h29m with 0 errors on Sun Jan  8 02:52:39 2017
config:

    NAME                                                        STATE     READ WRITE CKSUM
    fibrevillage02_1                                                   ONLINE       0     0     0
      raidz2-0                                                  ONLINE       0     0     0
        ata-HITACHI_H7220AA30SUN2.0T_0949M5EVJT_JK1130YAH5EVJT  ONLINE       0     0     0
        ata-HITACHI_H7220AA30SUN2.0T_0949M5BXKT_JK1130YAH5BXKT  ONLINE       0     0     0
        ata-HITACHI_H7220AA30SUN2.0T_0949M570WT_JK1130YAH570WT  ONLINE       0     0     0
        ata-HITACHI_H7220AA30SUN2.0T_0949M4Z9JT_JK1130YAH4Z9JT  ONLINE       0     0     0
        ata-HITACHI_H7220AA30SUN2.0T_0949M56NTT_JK1130YAH56NTT  ONLINE       0     0     0
        ata-HITACHI_H7220AA30SUN2.0T_0949M56MTT_JK1130YAH56MTT  ONLINE       0     0     0
        ata-HITACHI_H7220AA30SUN2.0T_0949MYJ48T_JK1130YAGYJ48T  ONLINE       0     0     0
        ata-HITACHI_H7220AA30SUN2.0T_0949M570RT_JK1130YAH570RT  ONLINE       0     0     0
        ata-HITACHI_H7220AA30SUN2.0T_0946MYG7ET_JK1130YAGYG7ET  ONLINE       0     0     0
        ata-HITACHI_H7220AA30SUN2.0T_0949M56V6T_JK1130YAH56V6T  ONLINE       0     0     0
        ata-HITACHI_H7220AA30SUN2.0T_0949M4ZNBT_JK1130YAH4ZNBT  ONLINE       0     0     0

errors: No known data errors

Repeat the steps for rest 3 ZFS pools

[root@fibrevillage02 ~]# zpool import fibrevillage02_2
[root@fibrevillage02 ~]# zpool import fibrevillage02_3
[root@fibrevillage02 ~]# zpool import fibrevillage02_4

ZFS pools upgrade(may not needed if ZFS versions are the same)

In the example above, you may noticed the following notice

status: Some supported features are not enabled on the pool. The pool can
    still be used, but some features are unavailable.
action: Enable all features using 'zpool upgrade'. Once this is done,
    the pool may no longer be accessible by software that does not support
    the features. See zpool-features(5) for details.

So, just run the upgrade to check

[root@fibrevillage02 ~]# zpool upgrade
This system supports ZFS pool feature flags.

All pools are formatted using feature flags.


Some supported features are not enabled on the following pools. Once a
feature is enabled the pool may become incompatible with software
that does not support the feature. See zpool-features(5) for details.

POOL  FEATURE
---------------
fibrevillage02_1
      spacemap_histogram
      enabled_txg
      hole_birth
      extensible_dataset
      embedded_data
      bookmarks
      filesystem_limits
      large_blocks
...

ZFS pool upgrade

[root@fibrevillage02 ~]# zpool upgrade fibrevillage02_1

This system supports ZFS pool feature flags.

Enabled the following features on 'fibrevillage02_1':
  spacemap_histogram
  enabled_txg
  hole_birth
  extensible_dataset
  embedded_data
  bookmarks
  filesystem_limits
  large_blocks

 Repeat the same steps for rest 3 ZFS pools

Check the upgrade status

# zpool upgrade -v
This system supports ZFS pool feature flags.

The following features are supported:

FEAT DESCRIPTION
-------------------------------------------------------------
async_destroy                         (read-only compatible)
     Destroy filesystems asynchronously.
empty_bpobj                           (read-only compatible)
     Snapshots use less space.
lz4_compress                         
     LZ4 compression algorithm support.
spacemap_histogram                    (read-only compatible)
     Spacemaps maintain space histograms.
enabled_txg                           (read-only compatible)
     Record txg at which a feature is enabled
hole_birth                           
     Retain hole birth txg for more precise zfs send
extensible_dataset                   
     Enhanced dataset functionality, used by other features.
embedded_data                        
     Blocks which compress very well use even less space.
bookmarks                             (read-only compatible)
     "zfs bookmark" command
filesystem_limits                     (read-only compatible)
     Filesystem and snapshot limits.
large_blocks                         
     Support for blocks larger than 128KB.

The following legacy versions are also supported:

VER  DESCRIPTION
---  --------------------------------------------------------
 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

For more information on a particular version, including supported releases,
see the ZFS Administration Guide.

All done!!