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 tpool pool: tpool 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 scrub: scrub completed after 0h0m with 2 errors on Tue Jul 13 11:08:37 2010 config: NAME STATE READ WRITE CKSUM tpool ONLINE 2 0 0 c1t1d0 ONLINE 2 0 0 c1t3d0 ONLINE 0 0 0 errors: Permanent errors have been detected in the following files: /tpool/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.