Onlining and Offlining Devices in a ZFS Storage Pool

Onlining and Offlining Devices in a ZFS Storage Pool

ZFS allows individual devices to be taken offline or brought online. When hardware is unreliable or not functioning properly, ZFS continues to read or write data to the device, assuming the condition is only temporary. If the condition is not temporary, it is possible to instruct ZFS to ignore the device by bringing it offline. ZFS does not send any requests to an offlined device.

Devices do not need to be taken offline in order to replace them.

You can use the offline command when you need to temporarily disconnect storage. For example, if you need to physically disconnect an array from one set of Fibre Channel switches and connect the array to a different set, you could take the LUNs offline from the array that was used in ZFS storage pools. After the array was reconnected and operational on the new set of switches, you could then bring the same LUNs online. Data that had been added to the storage pools while the LUNs were offline would resilver to the LUNs after they were brought back online.

This scenario is possible assuming that the systems in question see the storage once it is attached to the new switches, possibly through different controllers than before, and your pools are set up as RAID-Z or mirrored configurations.
Taking a Device Offline

You can take a device offline by using the zpool offline command. The device can be specified by path or by short name, if the device is a disk. For example:

# zpool offline tank c1t0d0
bringing device c1t0d0 offline

You cannot take a pool offline to the point where it becomes faulted. For example, you cannot take offline two devices out of a RAID-Z configuration, nor can you take offline a top-level virtual device.

# zpool offline tank c1t0d0
cannot offline c1t0d0: no valid replicas


Currently, you cannot replace a device that has been taken offline.

Offlined devices show up in the OFFLINE state when you query pool status. For information about querying pool status, see Querying ZFS Storage Pool Status.

By default, the offline state is persistent. The device remains offline when the system is rebooted.

To temporarily take a device offline, use the zpool offline t option. For example:

# zpool offline -t tank c1t0d0
bringing device ‘c1t0d0’ offline

When the system is rebooted, this device is automatically returned to the ONLINE state.

For more information on device health, see Health Status of ZFS Storage Pools.
Bringing a Device Online

Once a device is taken offline, it can be restored by using the zpool online command:

# zpool online tank c1t0d0
bringing device c1t0d0 online

When a device is brought online, any data that has been written to the pool is resynchronized to the newly available device. Note that you cannot use device onlining to replace a disk. If you offline a device, replace the drive, and try to bring it online, it remains in the faulted state.

If you attempt to online a faulted device, a message similar to the following is displayed from fmd:

# zpool online tank c1t0d0
Bringing device c1t0d0 online
SUNW-MSG-ID: ZFS-8000-D3, TYPE: Fault, VER: 1, SEVERITY: Major
EVENT-TIME: Fri Mar 17 14:38:47 MST 2006
SOURCE: zfs-diagnosis, REV: 1.0
EVENT-ID: 043bb0dd-f0a5-4b8f-a52d-8809e2ce2e0a
DESC: A ZFS device failed. Refer to for more information.
AUTO-RESPONSE: No automated response will occur.
IMPACT: Fault tolerance of the pool may be compromised.
REC-ACTION: Run ‘zpool status -x’ and replace the bad device.

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