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====== ZFS - Spares ======

ZFS has a special class of device known as a SPARE.

  * Unlike normal devices, a SPARE belongs to the entire pool, not a single vdev.
  * If any vdev in the pool suffers a device failure, and a SPARE is attached to the pool and available, the SPARE will automatically attach itself to the degraded vdev.

  * Once attached to the degraded vdev, the SPARE begins receiving copies or reconstructions of the data that should be on the missing device.
    * In traditional RAID, this would be called **rebuilding**.
    * In ZFS, this is called **resilvering**.

<WRAP info>
**IMPORTANT:**  SPARE devices do not __permanently__ replace failed devices.

  * They're just placeholders, intended to minimize the window during which a vdev runs degraded.
  * Once the admin has replaced the failed device of the vdev, and the new permanent replacement device resilvers, the SPARE detaches itself from the vdev, and returns to pool-wide duty.
</WRAP>