Resolving ZFS Storage Device Problems

Review the following sections to resolve a missing, removed or faulted device.

Resolving a Missing or Removed 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. If you need to replace a device that is UNAVAIL because it has failed, see Replacing a Device in a ZFS Storage Pool.

If a device is UNAVAIL in a root pool or a mirrored root pool, see the following references:

Mirrored root pool disk failed – Booting From an Alternate Disk in a Mirrored ZFS Root Pool
Replacing a disk in a root pool
- How to Replace a Disk in the ZFS Root Pool
Full root pool disaster recovery – Recovering the ZFS Root Pool or Root Pool Snapshots

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: daleks
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 http://sun.com/msg/ZFS-8000-FD 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'.
   see: http://www.sun.com/msg/ZFS-8000-2Q
  scan: scrub repaired 0 in 0h0m with 0 errors on Tue Sep 27 16:59:07 2011
config:

        NAME        STATE     READ WRITE CKSUM
        tank        DEGRADED     0     0     0
          mirror-0  DEGRADED     0     0     0
            c2t2d0  ONLINE       0     0     0
            c2t1d0  UNAVAIL      0     0     0  cannot open

errors: No known data errors

You can see from this output that the c2t1d0 device is not functioning. If you determine that the device is faulty, replace it.

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

# zpool online tank c2t1d0

Let FMA know that the device has been replaced if the output of the fmadm faulty identifies the device error. For example:

# fmadm faulty
--------------- ------------------------------------  -------------- ---------
TIME            EVENT-ID                              MSG-ID         SEVERITY
--------------- ------------------------------------  -------------- ---------
Sep 27 16:58:50 e6bb52c3-5fe0-41a1-9ccc-c2f8a6b56100  ZFS-8000-D3    Major     

Host        : neo
Platform    : SUNW,Sun-Fire-T200        Chassis_id  : 
Product_sn  : 

Fault class : fault.fs.zfs.device
Affects     : zfs://pool=tank/vdev=c75a8336cda03110
                  faulted and taken out of service
Problem in  : zfs://pool=tank/vdev=c75a8336cda03110
                  faulted and taken out of service

Description : A ZFS device failed.  Refer to http://sun.com/msg/ZFS-8000-D3 for
              more information.

Response    : No automated response will occur.

Impact      : Fault tolerance of the pool may be compromised.

Action      : Run 'zpool status -x' and replace the bad device.

# fmadm repaired zfs://pool=tank/vdev=c75a8336cda03110

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

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

Resolving a Removed Device

If a device is completely removed from the system, ZFS detects that the device cannot be opened and places it in the REMOVED state. Depending on the data replication level of the pool, this removal might or might not result in the entire pool becoming unavailable. If one disk in a mirrored or RAID-Z device is removed, the pool continues to be accessible. A pool might become UNAVAIL, which means no data is accessible until the device is reattached, under the following conditions:

If all components of a mirror are removed
If more than one device in a RAID-Z (raidz1) device is removed
If top-level device is removed in a single-disk configuration

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.

A pool might be SUSPENDED if device connectivity is compromised. A SUSPENDED pool remains in the wait state until the device issue is resolved. For example:

# zpool status cybermen
  pool: cybermen
 state: SUSPENDED
status: One or more devices are unavailable in response to IO failures.
        The pool is suspended.
action: Make sure the affected devices are connected, then run 'zpool clear' or
        'fmadm repaired'.
    see: http://www.sun.com/msg/ZFS-8000-HC
  scan: none requested
config:

        NAME           STATE     READ WRITE CKSUM
        cybermen       UNAVAIL      0    16     0
            c8t3d0     UNAVAIL      0     0     0
            c8t1d0     UNAVAIL      0     0     0

After device connectivity is restored, clear the pool or device errors.

# zpool clear cybermen
# fmadm repaired zfs://pool=name/vdev=guid

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 UNAVAIL or SUSPENDED, 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.

Replacing or Repairing a Damaged Device

This section describes how to determine device failure types, clear transient errors, and replacing a device.

Determining the Type of Device Failure

The term damaged device is rather vague and can describe a number of possible situations:

Bit rot – Over time, random events such as magnetic influences and cosmic rays can cause bits stored on disk to flip. These events are relatively rare but common enough to cause potential data corruption in large or long-running systems.
Misdirected reads or writes – Firmware bugs or hardware faults can cause reads or writes of entire blocks to reference the incorrect location on disk. These errors are typically transient, though a large number of them might indicate a faulty drive.
Administrator error – Administrators can unknowingly overwrite portions of a disk with bad data (such as copying /dev/zero over portions of the disk) that cause permanent corruption on disk. These errors are always transient.
Temporary outage– A disk might become unavailable for a period of time, causing I/Os to fail. This situation is typically associated with network-attached devices, though local disks can experience temporary outages as well. These errors might or might not be transient.
Bad or flaky hardware – This situation is a catch-all for the various problems that faulty hardware exhibits, including consistent I/O errors, faulty transports causing random corruption, or any number of failures. These errors are typically permanent.
Offline device – If a device is offline, it is assumed that the administrator placed the device in this state because it is faulty. The administrator who placed the device in this state can determine if this assumption is accurate.

Determining exactly what is wrong with a device can be a difficult process. The first step is to examine the error counts in the zpool status output. For example:

# zpool status -v tank
  pool: tank
 state: ONLINE
status: One or more devices has experienced an error resulting in data
        corruption.  Applications may be affected.
action: Restore the file in question if possible.  Otherwise restore the
        entire pool from backup.
   see: http://www.sun.com/msg/ZFS-8000-8A
  scan: scrub in progress since Tue Sep 27 17:12:40 2011
    63.9M scanned out of 528M at 10.7M/s, 0h0m to go
    0 repaired, 12.11% done
config:

        NAME        STATE     READ WRITE CKSUM
        tank        ONLINE       2     0     0
          mirror-0  ONLINE       2     0     0
            c2t2d0  ONLINE       2     0     0
            c2t1d0  ONLINE       2     0     0

errors: Permanent errors have been detected in the following files:

        /tank/words

The errors are divided into I/O errors and checksum errors, both of which might indicate the possible failure type. Typical operation predicts a very small number of errors (just a few over long periods of time). If you are seeing a large number of errors, then this situation probably indicates impending or complete device failure. However, an administrator error can also result in large error counts. The other source of information is the syslog system log. If the log shows a large number of SCSI or Fibre Channel driver messages, then this situation probably indicates serious hardware problems. If no syslog messages are generated, then the damage is likely transient.

The goal is to answer the following question:

Is another error likely to occur on this device?

Errors that happen only once are considered transient and do not indicate potential failure. Errors that are persistent or severe enough to indicate potential hardware failure are considered fatal. The act of determining the type of error is beyond the scope of any automated software currently available with ZFS, and so much must be done manually by you, the administrator. After determination is made, the appropriate action can be taken. Either clear the transient errors or replace the device due to fatal errors. These repair procedures are described in the next sections.

Even if the device errors are considered transient, they still might have caused uncorrectable data errors within the pool. These errors require special repair procedures, even if the underlying device is deemed healthy or otherwise repaired. For more information about repairing data errors, see Repairing Damaged Data.

Clearing Transient Device Errors

If the device errors are deemed transient, in that they are unlikely to affect the future health of the device, they can be safely cleared to indicate that no fatal error occurred. To clear error counters for RAID-Z or mirrored devices, use the zpool clear command. For example:

# zpool clear tank c1t1d0

This syntax clears any device errors and clears any data error counts associated with the device.

To clear all errors associated with the virtual devices in a pool, and to clear any data error counts associated with the pool, use the following syntax:

# zpool clear tank

For more information about clearing pool errors, see Clearing Storage Pool Device Errors.

Replacing a Device in a ZFS Storage Pool

If device damage is permanent or future permanent damage is likely, the device must be replaced. Whether the device can be replaced depends on the configuration.

Determining If a Device Can Be Replaced
Devices That Cannot be Replaced
Replacing a Device in a ZFS Storage Pool
Viewing Resilvering Status

Determining If a Device Can Be Replaced

If the device to be replaced is part of a redundant configuration, sufficient replicas from which to retrieve good data must exist. For example, if two disks in a four-way mirror are UNAVAIL, then either disk can be replaced because healthy replicas are available. However, if two disks in a four-way RAID-Z (raidz1) virtual device are UNAVAIL, then neither disk can be replaced because insufficient replicas from which to retrieve data exist. If the device is damaged but otherwise online, it can be replaced as long as the pool is not in the UNAVAIL state. However, any corrupted data on the device is copied to the new device, unless sufficient replicas with good data exist.

In the following configuration, the c1t1d0 disk can be replaced, and any data in the pool is copied from the healthy replica, c1t0d0:

    mirror            DEGRADED
    c1t0d0             ONLINE
    c1t1d0             FAULTED

The c1t0d0 disk can also be replaced, though no self-healing of data can take place because no good replica is available.

In the following configuration, neither UNAVAIL disk can be replaced. The ONLINE disks cannot be replaced either because the pool itself is UNAVAIL.

    raidz              FAULTED
    c1t0d0             ONLINE
    c2t0d0             FAULTED
    c3t0d0             FAULTED
    c4t0d0             ONLINE

In the following configuration, either top-level disk can be replaced, though any bad data present on the disk is copied to the new disk.

c1t0d0         ONLINE
c1t1d0         ONLINE

If either disk is UNAVAIL, then no replacement can be performed because the pool itself is UNAVAIL.

Devices That Cannot be Replaced

If the loss of a device causes the pool to become UNAVAIL or the device contains too many data errors in a non-redundant configuration, then the device cannot be safely replaced. Without sufficient redundancy, no good data with which to heal the damaged device exists. In this case, the only option is to destroy the pool and re-create the configuration, and then to restore your data from a backup copy.

For more information about restoring an entire pool, see Repairing ZFS Storage Pool-Wide Damage.

Replacing a Device in a ZFS Storage Pool

After you have determined that a device can be replaced, use the zpool replace command to replace the device. If you are replacing the damaged device with different device, use syntax similar to the following:

# zpool replace tank c1t1d0 c2t0d0

This command migrates data to the new device from the damaged device or from other devices in the pool if it is in a redundant configuration. When the command is finished, it detaches the damaged device from the configuration, at which point the device can be removed from the system. If you have already removed the device and replaced it with a new device in the same location, use the single device form of the command. For example:

# zpool replace tank c1t1d0

This command takes an unformatted disk, formats it appropriately, and then resilvers data from the rest of the configuration.

For more information about the zpool replace command, see Replacing Devices in a Storage Pool.

Example 10-1 Replacing a SATA Disk in a ZFS Storage Pool

The following example shows how to replace a device (c1t3d0) in a mirrored storage pool tank on a system with SATA devices. To replace the disk c1t3d0 with a new disk at the same location (c1t3d0), then you must unconfigure the disk before you attempt to replace it. If the disk to be replaced is not a SATA disk, then see Replacing Devices in a Storage Pool.

The basic steps follow:

Take offline the disk (c1t3d0)to be replaced. You cannot unconfigure a SATA disk that is currently being used.
Use the cfgadm command to identify the SATA disk (c1t3d0) to be unconfigured and unconfigure it. The pool will be degraded with the offline disk in this mirrored configuration, but the pool will continue to be available.
Physically replace the disk (c1t3d0). Ensure that the blue Ready to Remove LED is illuminated before you physically remove the UNAVAIL drive, if available.
Reconfigure the SATA disk (c1t3d0).
Bring the new disk (c1t3d0) online.
Run the zpool replace command to replace the disk (c1t3d0).

Note - If you had previously set the pool property autoreplace to on, then any new device, found in the same physical location as a device that previously belonged to the pool is automatically formatted and replaced without using the zpool replace command. This feature might not be supported on all hardware.
If a failed disk is automatically replaced with a hot spare, you might need to detach the hot spare after the failed disk is replaced. For example, if c2t4d0 is still an active hot spare after the failed disk is replaced, then detach it.
```
# zpool detach tank c2t4d0
```
If FMA is reporting the failed device, then you should clear the device failure.
```
# fmadm faulty
# fmadm repaired zfs://pool=name/vdev=guid
```

The following example walks through the steps to replace a disk in a ZFS storage pool.

# zpool offline tank c1t3d0
# cfgadm | grep c1t3d0
sata1/3::dsk/c1t3d0            disk         connected    configured   ok
# cfgadm -c unconfigure sata1/3
Unconfigure the device at: /devices/pci@0,0/pci1022,7458@2/pci11ab,11ab@1:3
This operation will suspend activity on the SATA device
Continue (yes/no)? yes
# cfgadm | grep sata1/3
sata1/3                        disk         connected    unconfigured ok
<Physically replace the failed disk c1t3d0>
# cfgadm -c configure sata1/3
# cfgadm | grep sata1/3
sata1/3::dsk/c1t3d0            disk         connected    configured   ok
# zpool online tank c1t3d0
# zpool replace tank c1t3d0
# zpool status tank
  pool: tank
 state: ONLINE
 scrub: resilver completed after 0h0m with 0 errors on Tue Feb  2 13:17:32 2010
config:

        NAME        STATE     READ WRITE CKSUM
        tank        ONLINE       0     0     0
          mirror-0  ONLINE       0     0     0
            c0t1d0  ONLINE       0     0     0
            c1t1d0  ONLINE       0     0     0
          mirror-1  ONLINE       0     0     0
            c0t2d0  ONLINE       0     0     0
            c1t2d0  ONLINE       0     0     0
          mirror-2  ONLINE       0     0     0
            c0t3d0  ONLINE       0     0     0
            c1t3d0  ONLINE       0     0     0

errors: No known data errors

Note that the preceding zpool output might show both the new and old disks under a replacing heading. For example:

replacing     DEGRADED     0     0    0
  c1t3d0s0/o  FAULTED      0     0    0
  c1t3d0      ONLINE       0     0    0

This text means that the replacement process is in progress and the new disk is being resilvered.

If you are going to replace a disk (c1t3d0) with another disk (c4t3d0), then you only need to run the zpool replace command. For example:

# zpool replace tank c1t3d0 c4t3d0
# zpool status
  pool: tank
 state: DEGRADED
 scrub: resilver completed after 0h0m with 0 errors on Tue Feb  2 13:35:41 2010
config:

        NAME             STATE     READ WRITE CKSUM
        tank             DEGRADED     0     0     0
          mirror-0       ONLINE       0     0     0
            c0t1d0       ONLINE       0     0     0
            c1t1d0       ONLINE       0     0     0
          mirror-1       ONLINE       0     0     0
            c0t2d0       ONLINE       0     0     0
            c1t2d0       ONLINE       0     0     0
          mirror-2       DEGRADED     0     0     0
            c0t3d0       ONLINE       0     0     0
            replacing    DEGRADED     0     0     0
              c1t3d0     OFFLINE      0     0     0
              c4t3d0     ONLINE       0     0     0

errors: No known data errors

You might need to run the zpool status command several times until the disk replacement is completed.

# zpool status tank
  pool: tank
 state: ONLINE
 scrub: resilver completed after 0h0m with 0 errors on Tue Feb  2 13:35:41 2010
config:

        NAME          STATE     READ WRITE CKSUM
        tank          ONLINE       0     0     0
          mirror-0    ONLINE       0     0     0
            c0t1d0    ONLINE       0     0     0
            c1t1d0    ONLINE       0     0     0
          mirror-1    ONLINE       0     0     0
            c0t2d0    ONLINE       0     0     0
            c1t2d0    ONLINE       0     0     0
          mirror-2    ONLINE       0     0     0
            c0t3d0    ONLINE       0     0     0
            c4t3d0    ONLINE       0     0     0

Example 10-2 Replacing a Failed Log Device

ZFS identifies intent log failures in the zpool status command output. Fault Management Architecture (FMA) reports these errors as well. Both ZFS and FMA describe how to recover from an intent log failure.

The following example shows how to recover from a failed log device (c0t5d0) in the storage pool (pool). The basic steps follow:

Review the zpool status -x output and FMA diagnostic message, described here:

https://support.oracle.com/CSP/main/article?cmd=show&type=NOT&doctype=REFERENCE&alias=EVENT:ZFS-8000-K4
Physically replace the failed log device.
Bring the new log device online.
Clear the pool's error condition.
Clear the FMA error.

# zpool status -x
  pool: pool
 state: FAULTED
status: One or more of the intent logs could not be read.
        Waiting for adminstrator intervention to fix the faulted pool.
action: Either restore the affected device(s) and run 'zpool online',
        or ignore the intent log records by running 'zpool clear'.
 scrub: none requested
config:

        NAME        STATE     READ WRITE CKSUM
        pool        FAULTED      0     0     0 bad intent log
          mirror    ONLINE       0     0     0
            c0t1d0  ONLINE       0     0     0
            c0t4d0  ONLINE       0     0     0
        logs        FAULTED      0     0     0 bad intent log
          c0t5d0    UNAVAIL      0     0     0 cannot open
<Physically replace the failed log device>
# zpool online pool c0t5d0
# zpool clear pool

For example, if the system shuts down abruptly before synchronous write operations are committed to a pool with a separate log device, you see messages similar to the following:

# zpool status -x
  pool: pool
 state: FAULTED
status: One or more of the intent logs could not be read.
        Waiting for adminstrator intervention to fix the faulted pool.
action: Either restore the affected device(s) and run 'zpool online',
        or ignore the intent log records by running 'zpool clear'.
 scrub: none requested
config:

        NAME          STATE     READ WRITE CKSUM
        pool          FAULTED      0     0     0 bad intent log
          mirror-0    ONLINE       0     0     0
            c0t1d0    ONLINE       0     0     0
            c0t4d0    ONLINE       0     0     0
        logs          FAULTED      0     0     0 bad intent log
          c0t5d0      UNAVAIL      0     0     0 cannot open
<Physically replace the failed log device>
# zpool online pool c0t5d0
# zpool clear pool
# fmadm faulty
# fmadm repair zfs://pool=name/vdev=guid

You can resolve the log device failure in the following ways:

Replace or recover the log device. In this example, the log device is c0t5d0.
Bring the log device back online.
```
# zpool online pool c0t5d0
```
Reset the failed log device error condition.
```
# zpool clear pool
```

To recover from this error without replacing the failed log device, you can clear the error with the zpool clear command. In this scenario, the pool will operate in a degraded mode and the log records will be written to the main pool until the separate log device is replaced.

Consider using mirrored log devices to avoid the log device failure scenario.

Viewing Resilvering Status

The process of replacing a device can take an extended period of time, depending on the size of the device and the amount of data in the pool. The process of moving data from one device to another device is known as resilvering and can be monitored by using the zpool status command.

Traditional file systems resilver data at the block level. Because ZFS eliminates the artificial layering of the volume manager, it can perform resilvering in a much more powerful and controlled manner. The two main advantages of this feature are as follows:

ZFS only resilvers the minimum amount of necessary data. In the case of a short outage (as opposed to a complete device replacement), the entire disk can be resilvered in a matter of minutes or seconds. When an entire disk is replaced, the resilvering process takes time proportional to the amount of data used on disk. Replacing a 500-GB disk can take seconds if a pool has only a few gigabytes of used disk space.
Resilvering is interruptible and safe. If the system loses power or is rebooted, the resilvering process resumes exactly where it left off, without any need for manual intervention.

To view the resilvering process, use the zpool status command. For example:

# zpool status tank
  pool: tank
 state: DEGRADED
status: One or more devices is currently being resilvered.  The pool will
        continue to function, possibly in a degraded state.
action: Wait for the resilver to complete.
 scrub: resilver in progress for 0h0m, 22.60% done, 0h1m to go
config:
        NAME                  STATE     READ WRITE CKSUM 
        tank             DEGRADED     0     0     0
          mirror-0       DEGRADED     0     0     0
            replacing-0  DEGRADED     0     0     0
              c1t0d0     UNAVAIL      0     0     0  cannot open
              c2t0d0     ONLINE       0     0     0  85.0M resilvered
            c1t1d0       ONLINE       0     0     0

errors: No known data errors

In this example, the disk c1t0d0 is being replaced by c2t0d0. This event is observed in the status output by the presence of the replacing virtual device in the configuration. This device is not real, nor is it possible for you to create a pool by using it. The purpose of this device is solely to display the resilvering progress and to identify which device is being replaced.

Note that any pool currently undergoing resilvering is placed in the ONLINE or DEGRADED state because the pool cannot provide the desired level of redundancy until the resilvering process is completed. Resilvering proceeds as fast as possible, though the I/O is always scheduled with a lower priority than user-requested I/O, to minimize impact on the system. After the resilvering is completed, the configuration reverts to the new, complete, configuration. For example:

# zpool status tank
  pool: tank
 state: ONLINE
 scrub: resilver completed after 0h1m with 0 errors on Tue Feb  2 13:54:30 2010
config:

        NAME        STATE     READ WRITE CKSUM
        tank        ONLINE       0     0     0
          mirror-0  ONLINE       0     0     0
            c2t0d0  ONLINE       0     0     0  377M resilvered
            c1t1d0  ONLINE       0     0     0

errors: No known data errors

The pool is once again ONLINE, and the original failed disk (c1t0d0) has been removed from the configuration.

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