Checking ZFS Data Integrity

No fsck utility equivalent exists for ZFS. This utility has traditionally served two purposes, data repair and data validation.

Data Repair

With traditional file systems, the way in which data is written is inherently vulnerable to unexpected failure causing data inconsistencies. Because a traditional file system is not transactional, unreferenced blocks, bad link counts, or other inconsistent data structures are possible. The addition of journaling does solve some of these problems, but can introduce additional problems when the log cannot be rolled back. With ZFS, none of these problems exist. The only way for inconsistent data to exist on disk is through hardware failure (in which case the pool should have been replicated) or a bug in the ZFS software exists.

Given that the fsck utility is designed to repair known pathologies specific to individual file systems, writing such a utility for a file system with no known pathologies is impossible. Future experience might prove that certain data corruption problems are common enough and simple enough such that a repair utility can be developed, but these problems can always be avoided by using replicated pools.

If your pool is not replicated, the chance that data corruption can render some or all of your data inaccessible is always represent.

Data Validation

In addition to data repair, the fsck utility validates that the data on disk has no problems. Traditionally, this task is done by unmounting the file system and running the fsck utility, possibly taking the system to single-user mode in the process. This scenario results in downtime that is proportional to the size of the file system being checked. Instead of requiring an explicit utility to perform the necessary checking, ZFS provides a mechanism to perform regular checking of all data. This functionality, known as scrubbing, is commonly used in memory and other systems as a method of detecting and preventing errors before they result in hardware or software failure.

Controlling ZFS Data Scrubbing

Whenever ZFS encounters an error, either through scrubbing or when accessing a file on demand, the error is logged internally so that you can get a quick overview of all known errors within the pool.

Explicit ZFS Data Scrubbing

The simplest way to check your data integrity is to initiate an explicit scrubbing of all data within the pool. This operation traverses all the data in the pool once and verifies that all blocks can be read. Scrubbing proceeds as fast as the devices allow, though the priority of any I/O remains below that of normal operations. This operation might negatively impact performance, though the file system should remain usable and nearly as responsive while the scrubbing occurs. To initiate an explicit scrub, use the zpool scrub command. For example:

# zpool scrub tank

The status of the current scrub can be displayed in the zpool status output. For example:

# zpool status -v tank
  pool: tank
 state: ONLINE
 scrub: scrub completed with 0 errors on Tue Mar  7 15:27:36 2006

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

errors: No known data errors

Note that only one active scrubbing operation per pool can occur at one time.

Performing regular scrubbing also guarantees continuous I/O to all disks on the system. Regular scrubbing has the side effect of preventing power management from placing idle disks in low-power mode. If the system is generally performing I/O all the time, or if power consumption is not a concern, then this issue can safely be ignored.

For more information about interpreting zpool status output, see Querying ZFS Storage Pool Status.

ZFS Data Scrubbing and Resilvering

When a device is replaced, a resilvering operation is initiated to move data from the good copies to the new device. This action is a form of disk scrubbing. Therefore, only one such action can happen at a given time in the pool. If a scrubbing operation is in progress, a resilvering operation suspends the current scrubbing, and restarts after the resilvering is complete.

For more information about resilvering, see Viewing Resilvering Status.