Oracle Solaris ZFS Administration Guide

Onlining and Offlining Devices in a 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 data from or write data to the device, assuming the condition is only temporary. If the condition is not temporary, you can instruct ZFS to ignore the device by taking it offline. ZFS does not send any requests to an offline device.

Note –

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

You can use the zpool 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 can take offline the LUNs from the array that is used in the ZFS storage pools. After the array is reconnected and operational on the new set of switches, you can 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 are brought back online.

This scenario is possible assuming that the systems in question can detect the storage after 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

Consider the following points when taking a device offline:

You cannot take a pool offline to the point where it becomes faulted. For example, you cannot take offline two devices in a raidz1 configuration, nor can you take offline a top-level virtual device.
# zpool offline tank c1t0d0 cannot offline c1t0d0: no valid replicas
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.
When a device is taken offline, it is not detached from the storage pool. If you attempt to use the offline device in another pool, even after the original pool is destroyed, you see a message similar to the following:
device is part of exported or potentially active ZFS pool. Please see zpool(1M)
If you want to use the offline device in another storage pool after destroying the original storage pool, first bring the device online, then destroy the original storage pool.

Another way to use a device from another storage pool, while keeping the original storage pool, is to replace the existing device in the original storage pool with another comparable device. For information about replacing devices, see Replacing Devices in a Storage Pool.

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

For more information on device health, see Determining the Health Status of ZFS Storage Pools.

Bringing a Device Online

After a device is taken offline, it can be brought online again by using the zpool online command. For example:

# 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 with the newly available device. Note that you cannot use bring a device online to replace a disk. If you take a device offline, replace the device, and try to bring it online, it remains in the faulted state.

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

# zpool online tank c1t0d0
warning: device 'c1t0d0' onlined, but remains in faulted state
use 'zpool replace' to replace devices that are no longer present

You might also see the faulted disk message displayed on the console or written to the /var/adm/messages file. For example:

SUNW-MSG-ID: ZFS-8000-D3, TYPE: Fault, VER: 1, SEVERITY: Major
EVENT-TIME: Wed Jun 30 14:53:39 MDT 2010
PLATFORM: SUNW,Sun-Fire-880, CSN: -, HOSTNAME: neo
SOURCE: zfs-diagnosis, REV: 1.0
EVENT-ID: 504a1188-b270-4ab0-af4e-8a77680576b8
DESC: A ZFS device failed.  Refer to http://sun.com/msg/ZFS-8000-D3 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.

For more information about replacing a faulted device, see Resolving a Missing Device.

You can use the zpool online -e command to expand the pool's size when a smaller disk is replaced by a larger disk. By default, pool size is not expanded to its full size unless the autoexpand pool property is enabled. You can expand the pool size automatically when a smaller disk is replaced by a larger disk by using the zpool online -ecommand even if the disk is already online or if the disk is currently offline. For example:

# zpool online -e tank c1t13d0