Copy on Write (COW) is a fundamental underpinning beneath most of what makes ZFS awesome.

• When a file is modified, the copy-on-write filesystem writes out a new version of the block that is modified, then updates the metadata of the files to unlink the old block, and link the new block just written.
• Unlinking the old block and linking in the new is accomplished in a single operation, so it cannot be interrupted.
  • If the power went down after it happens, you have the new version of the file, and if the power went off before, then you have the old version.
  • Either way, the filesystem is consistent.

Copy-on-write in ZFS is not only at the filesystem level.

• It is also at the disk management level.
• This means that the RAID hole, a condition in which a stripe is only partially written before the system crashes, making the array inconsistent and corrupt after a restart, does not affect ZFS.
• Stripe writes are atomic, the vdev is always consistent.