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====== ZFS - Structure - vdev ======

Each zpool consists of one or more vdevs (virtual devices).

  * Each vdev, in turn, consists of one or more real devices.
  * Most vdevs are used for plain storage, but several special support classes of vdev exist:
    * CACHE
    * LOG
    * SPECIAL
  * Each of these vdev types can offer one of five topologies
    * Single-device.
    * Mirror.
    * RAIDz1. 
    * RAIDz2.
    * RAIDz3.

RAIDz1, RAIDz2, and RAIDz3 are special varieties of what storage greybeards call "diagonal parity RAID." 

  * The 1, 2, and 3 refer to how many parity blocks are allocated to each data stripe.
    * Rather than having entire disks dedicated to parity, RAIDz vdevs distribute that parity semi-evenly across the disks.
  * A RAIDz array can lose as many disks as it has parity blocks; if it loses another, it fails, and takes the zpool down with it.

Mirror vdevs mirror each block on every device in the vdev.

  * Although two-wide mirrors are the most common, a mirror vdev can contain any arbitrary number of devices—three-way are common in larger setups for the higher read performance and fault resistance.
  * A mirror vdev can survive any failure, so long as at least one device in the vdev remains healthy.

Single-device vdevs are inherently dangerous.

  * A single-device vdev cannot survive any failure.
  * If it is being used as a storage or SPECIAL vdev, its failure will take the entire zpool down with it.
  * Be very, very careful here!!!

CACHE, LOG, and SPECIAL vdevs can be created using any of the above topologies.

  * But remember, loss of a SPECIAL vdev means loss of the pool, so redundant topology is strongly encouraged.