Repairing a Damaged Device
This section describes how to determine device failure types, clear transient errors, and replace a device.
Determining the Type of Device Failure
Determining exactly what is wrong can be a difficult process. The first step is to examine the error counts in the zpool status output as follows:
# zpool status -v pool
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 large numbers of errors, then this situation probably indicates impending or complete device failure. However, the pathology for administrator error can result in large error counts. The other source of information is the 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. Once the 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, it still may 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 on repairing data errors, see Repairing Damaged Data.
Clearing Transient Errors
If the device errors are deemed transient, in that they are unlikely to effect the future health of the device, then the device errors 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 c1t0d0
This syntax clears any errors associated with the device and clears any data error counts associated with the device.
To clear all errors associated with the virtual devices in the pool, and 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 Devices.
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
For a device to be replaced, the pool must be in the ONLINE state. The device must be part of a redundant configuration, or it must be healthy (in the ONLINE state). If the disk is part of a redundant configuration, sufficient replicas from which to retrieve good data must exist. If two disks in a four-way mirror are faulted, then either disk can be replaced because healthy replicas are available. However, if two disks in a four-way RAID-Z device are faulted, then neither disk can be replaced because not enough 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 FAULTED state. However, any bad data on the device is copied to the new device unless there are sufficient replicas with good data.
In the following configuration, the disk c1t1d0 can be replaced, and any data in the pool is copied from the good replica, c1t0d0.
mirror DEGRADED c1t0d0 ONLINE c1t1d0 FAULTED
The disk c1t0d0 can also be replaced, though no self-healing of data can take place because no good replica is available.
In the following configuration, neither of the faulted disks can be replaced. The ONLINE disks cannot be replaced either, because the pool itself is faulted.
raidz FAULTED c1t0d0 ONLINE c2t0d0 FAULTED c3t0d0 FAULTED c3t0d0 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 were faulted, then no replacement could be performed because the pool itself would be faulted.
Devices That Cannot be Replaced
If the loss of a device causes the pool to become faulted, or the device contains too many data errors in an non-redundant configuration, then the device cannot safely be 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, restoring your data in the process.
For more information about restoring an entire pool, see Repairing ZFS Storage Pool-Wide Damage.
Replacing a Device in a ZFS Storage Pool
Once 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 another different device, use the following command:
# zpool replace tank c1t0d0 c2t0d0
This command begins migrating data to the new device from the damaged device, or 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 c1t0d0
This command takes an unformatted disk, formats it appropriately, and then begins resilvering data from the rest of the configuration.
For more information about the zpool replace command, see Replacing Devices in a Storage Pool.
Viewing Resilvering Status
The process of replacing a drive can take an extended period of time, depending on the size of the drive 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:
To view the resilvering process, use the zpool status command. For example:
# zpool status tank pool: tank state: DEGRADED reason: One or more devices is being resilvered. action: Wait for the resilvering process to complete. see: http://www.sun.com/msg/ZFS-XXXX-08 scrub: none requested config: NAME STATE READ WRITE CKSUM tank DEGRADED 0 0 0 mirror DEGRADED 0 0 0 replacing DEGRADED 0 0 0 52% resilvered c1t0d0 ONLINE 0 0 0 c2t0d0 ONLINE 0 0 0 c1t1d0 ONLINE 0 0 0
In this example, the disk c1t0d0 is being replaced by c2t0d0. This event is observed in the status output by 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 this virtual device type. The purpose of this device is solely to display the resilvering process, and to identify exactly which device is being replaced.
Note that any pool currently undergoing resilvering is placed in the DEGRADED state, because the pool cannot provide the desired level of redundancy until the resilvering process is complete. 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. Once the resilvering is complete, the configuration reverts to the new, complete, configuration. For example:
# zpool status tank pool: tank state: ONLINE scrub: scrub completed with 0 errors on Thu Aug 31 11:20:18 2006 config: NAME STATE READ WRITE CKSUM tank ONLINE 0 0 0 mirror ONLINE 0 0 0 c2t0d0 ONLINE 0 0 0 c1t1d0 ONLINE 0 0 0 errors: No known data errors
The pool is once again ONLINE, and the original bad disk (c1t0d0) has been removed from the configuration.