Onlining and Offlining Devices in a Storage Pool (Solaris ZFS Administration Guide)

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 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.

Note –

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

Keep the following points in mind 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 out of 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 offlined device in another pool, even after the original pool is destroyed, you will 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 offlined device in another storage pool after destroying the original storage pool, first bring the device back online, then destroy the original storage pool.

Another way to use a device from another storage pool if you want to keep 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.

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.

For more information on device health, see Determining the 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:

# 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 from fmd.

SUNW-MSG-ID: ZFS-8000-D3, TYPE: Fault, VER: 1, SEVERITY: Major
EVENT-TIME: Fri Aug 28 14:08:39 MDT 2009
PLATFORM: SUNW,Sun-Fire-T200, CSN: -, HOSTNAME: neo2
SOURCE: zfs-diagnosis, REV: 1.0
EVENT-ID: 9da778a7-a828-c88a-d679-c9a7873f4808
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 on replacing a faulted device, see Resolving a Missing Device.

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

# zpool online -e tank c1t13d0