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 after 0h7m with 0 errors on Tue Sep  1 09:20:52 2009
config:

        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

Only one active scrubbing operation per pool can occur at one time.

You can stop a scrub that is in progress by using the -s option. For example:

# zpool scrub -s tank

In most cases, a scrub operation, to ensure data integrity, should continue to completion. Stop a scrub at your own discretion if system performance is impacted by a scrub operation.

Performing routine scrubbing also guarantees continuous I/O to all disks on the system. Routine 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 it after the resilvering is complete.

For more information about resilvering, see Viewing Resilvering Status.