Oracle Solaris ZFS Administration Guide

Resolving a Missing Device

If a device cannot be opened, it displays the UNAVAIL state in the zpool status output. This state means that ZFS was unable to open the device when the pool was first accessed, or the device has since become unavailable. If the device causes a top-level virtual device to be unavailable, then nothing in the pool can be accessed. Otherwise, the fault tolerance of the pool might be compromised. In either case, the device just needs to be reattached to the system to restore normal operations.

For example, you might see a message similar to the following from fmd after a device failure:

SUNW-MSG-ID: ZFS-8000-FD, TYPE: Fault, VER: 1, SEVERITY: Major
EVENT-TIME: Thu Jun 24 10:42:36 PDT 2010
PLATFORM: SUNW,Sun-Fire-T200, CSN: -, HOSTNAME: neo2
SOURCE: zfs-diagnosis, REV: 1.0
EVENT-ID: a1fb66d0-cc51-cd14-a835-961c15696fcb
DESC: The number of I/O errors associated with a ZFS device exceeded
acceptable levels.  Refer to for more information.
AUTO-RESPONSE: The device has been offlined and marked as faulted.  An attempt
will be made to activate a hot spare if available. 
IMPACT: Fault tolerance of the pool may be compromised.
REC-ACTION: Run 'zpool status -x' and replace the bad device.

To view more detailed information about the device problem and the resolution, use the zpool status -x command. For example:

# zpool status -x
  pool: tank
 state: DEGRADED
status: One or more devices could not be opened.  Sufficient replicas exist for
        the pool to continue functioning in a degraded state.
action: Attach the missing device and online it using 'zpool online'.
 scrub: scrub completed after 0h0m with 0 errors on Tue Feb  2 13:15:20 2010

        NAME        STATE     READ WRITE CKSUM
        tank        DEGRADED     0     0     0
          mirror-0  DEGRADED     0     0     0
            c1t0d0  ONLINE       0     0     0
            c1t1d0  UNAVAIL      0     0     0  cannot open

errors: No known data errors

You can see from this output that the missing c1t1d0 device is not functioning. If you determine that the device is faulty, replace it.

Then, use the zpool online command to bring online the replaced device. For example:

# zpool online tank c1t1d0

As a last step, confirm that the pool with the replaced device is healthy. For example:

# zpool status -x tank
pool 'tank' is healthy

Physically Reattaching a Device

Exactly how a missing device is reattached depends on the device in question. If the device is a network-attached drive, connectivity to the network should be restored. If the device is a USB device or other removable media, it should be reattached to the system. If the device is a local disk, a controller might have failed such that the device is no longer visible to the system. In this case, the controller should be replaced, at which point the disks will again be available. Other problems can exist and depend on the type of hardware and its configuration. If a drive fails and it is no longer visible to the system, the device should be treated as a damaged device. Follow the procedures in Replacing or Repairing a Damaged Device.

Notifying ZFS of Device Availability

After a device is reattached to the system, ZFS might or might not automatically detect its availability. If the pool was previously faulted, or the system was rebooted as part of the attach procedure, then ZFS automatically rescans all devices when it tries to open the pool. If the pool was degraded and the device was replaced while the system was running, you must notify ZFS that the device is now available and ready to be reopened by using the zpool online command. For example:

# zpool online tank c0t1d0

For more information about bringing devices online, see Bringing a Device Online.