Implement shadow mirrors

[pcd1193182]

Sponsored by: Railway Corporation; Klara, Inc.

Description

Shadow mirrors provide a mechanism for adding an experimental or untrustworthy device to ZFS. This can be useful for testing purposes, where we want to put actual load on a drive in a production or near-production environment, but also want the assurances of existing, reliable storage. Shadow mirrors issue reads to all of their shadow children and one of their non-shadow children (like regular mirrors). The shadow reads are checked for correctness, but not used to return the result of the read. Writes are directed to all children as usual.

How Has This Been Tested?

In addition to the new unit test, manual testing was performed to ensure that reads and writes are directed appropriately (all reads go to the shadow child, non-shadow children are distributed as normal).

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Quality assurance (non-breaking change which makes the code more robust against bugs)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.