Identifying Problems in ZFS

All ZFS troubleshooting is centered around the zpool status command. This command analyzes the various failures in the system and identifies the most severe problem, presenting you with a suggested action and a link to a knowledge article for more information. Note that the command only identifies a single problem with the pool, though multiple problems can exist. For example, data corruption errors always imply that one of the devices has failed. Replacing the failed device does not fix the data corruption problems.

In addition, a ZFS diagnostic engine is provided to diagnose and report pool failures and device failures. Checksum, I/O, device, and pool errors associated with pool or device failures are also reported. ZFS failures as reported by fmd are displayed on the console as well as the system messages file. In most cases, the fmd message directs you to the zpool status command for further recovery instructions.

The basic recovery process is as follows:

This chapter describes how to interpret zpool status output in order to diagnose the type of failure and directs you to one of the following sections on how to repair the problem. While most of the work is performed automatically by the command, it is important to understand exactly what problems are being identified in order to diagnose the type of failure.

Determining if Problems Exist in a ZFS Storage Pool

The easiest way to determine if any known problems exist on the system is to use the zpool status x command. This command describes only pools exhibiting problems. If no bad pools exist on the system, then the command displays a simple message, as follows:

# zpool status -x
all pools are healthy

Without the x flag, the command displays the complete status for all pools (or the requested pool, if specified on the command line), even if the pools are otherwise healthy.

For more information about command-line options to the zpool status command, see Querying ZFS Storage Pool Status.

Understanding zpool status Output

The complete zpool status output looks similar to the following:

# zpool status tank
  pool: tank
 state: DEGRADED
status: One or more devices has been taken offline by the administrator.
        Sufficient replicas exist for the pool to continue functioning in a
        degraded state.
action: Online the device using 'zpool online' or replace the device with
        'zpool replace'.
 scrub: none requested

        NAME         STATE     READ WRITE CKSUM
        tank         DEGRADED     0     0     0
          mirror     DEGRADED     0     0     0
            c1t0d0   ONLINE       0     0     0
            c1t1d0   OFFLINE      0     0     0

errors: No known data errors

This output is divided into several sections:

Overall Pool Status Information

This header section in the zpool status output contains the following fields, some of which are only displayed for pools exhibiting problems:


The name of the pool.


The current health of the pool. This information refers only to the ability of the pool to provide the necessary replication level. Pools that are ONLINE might still have failing devices or data corruption.


A description of what is wrong with the pool. This field is omitted if no problems are found.


A recommended action for repairing the errors. This field is an abbreviated form directing the user to one of the following sections. This field is omitted if no problems are found.


A reference to a knowledge article containing detailed repair information. Online articles are updated more often than this guide can be updated, and should always be referenced for the most up-to-date repair procedures. This field is omitted if no problems are found.


Identifies the current status of a scrub operation, which might include the date and time that the last scrub was completed, a scrub in progress, or if no scrubbing was requested.


Identifies known data errors or the absence of known data errors.

Configuration Information

The config field in the zpool status output describes the configuration layout of the devices comprising the pool, as well as their state and any errors generated from the devices. The state can be one of the following: ONLINE, FAULTED, DEGRADED, UNAVAILABLE, or OFFLINE. If the state is anything but ONLINE, the fault tolerance of the pool has been compromised.

The second section of the configuration output displays error statistics. These errors are divided into three categories:

  • READ – I/O error occurred while issuing a read request.

  • WRITE – I/O error occurred while issuing a write request.

  • CKSUM – Checksum error. The device returned corrupted data as the result of a read request.

These errors can be used to determine if the damage is permanent. A small number of I/O errors might indicate a temporary outage, while a large number might indicate a permanent problem with the device. These errors do not necessarily correspond to data corruption as interpreted by applications. If the device is in a redundant configuration, the disk devices might show uncorrectable errors, while no errors appear at the mirror or RAID-Z device level. If this scenario is the case, then ZFS successfully retrieved the good data and attempted to heal the damaged data from existing replicas.

For more information about interpreting these errors to determine device failure, see Determining the Type of Device Failure.

Finally, additional auxiliary information is displayed in the last column of the zpool status output. This information expands on the state field, aiding in diagnosis of failure modes. If a device is FAULTED, this field indicates whether the device is inaccessible or whether the data on the device is corrupted. If the device is undergoing resilvering, this field displays the current progress.

For more information about monitoring resilvering progress, see Viewing Resilvering Status.

Scrubbing Status

The third section of the zpool status output describes the current status of any explicit scrubs. This information is distinct from whether any errors are detected on the system, though this information can be used to determine the accuracy of the data corruption error reporting. If the last scrub ended recently, most likely, any known data corruption has been discovered.

For more information about data scrubbing and how to interpret this information, see Checking ZFS Data Integrity.

Data Corruption Errors

The zpool status command also shows whether any known errors are associated with the pool. These errors might have been found during disk scrubbing or during normal operation. ZFS maintains a persistent log of all data errors associated with the pool. This log is rotated whenever a complete scrub of the system finishes.

Data corruption errors are always fatal. Their presence indicates that at least one application experienced an I/O error due to corrupt data within the pool. Device errors within a replicated pool do not result in data corruption and are not recorded as part of this log. By default, only the number of errors found is displayed. A complete list of errors and their specifics can be found by using the zpool status v option. For example:

# zpool status -v
  pool: tank
 state: DEGRADED
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.
 scrub: resilver completed with 1 errors on Fri Mar 17 15:42:18 2006

        NAME         STATE     READ WRITE CKSUM
        tank         DEGRADED     0     0     1
          mirror     DEGRADED     0     0     1
            c1t0d0   ONLINE       0     0     2
            c1t1d0   UNAVAIL      0     0     0  corrupted data

errors: The following persistent errors have been detected:

          5        0       lvl=4294967295 blkid=0

A similar message is also displayed by fmd on the system console and the /var/adm/messages file. These messages can also be tracked by using the fmdump command.

For more information about interpreting data corruption errors, see Identifying the Type of Data Corruption.

System Reporting of ZFS Error Messages

In addition to persistently keeping track of errors within the pool, ZFS also displays syslog messages when events of interest occur. The following scenarios generate events to notify the administrator:

  • Device state transition – If a device becomes FAULTED, ZFS logs a message indicating that the fault tolerance of the pool might be compromised. A similar message is sent if the device is later brought online, restoring the pool to health.

  • Data corruption – If any data corruption is detected, ZFS logs a message describing when and where the corruption was detected. This message is only logged the first time it is detected. Subsequent accesses do not generate a message.

  • Pool failures and device failures – If a pool failure or device failure occurs, the fault manager daemon reports these errors through syslog messages as well as the fmdump command.

If ZFS detects a device error and automatically recovers from it, no notification occurs. Such errors do not constitute a failure in the pool redundancy or data integrity. Moreover, such errors are typically the result of a driver problem accompanied by its own set of error messages.