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Having a copy of repository content on disk may enable a preservation workflow, but it is not a preservation strategy by itself.  So transparent persistence is "preservation-enabling", allowing a disk-based preservation workflow to easily access the repository content.

Existing functionality

• fcrepo-message-consumer (deprecated)serialization can be configured to serialize metadata as RDF files on disk using the RdfPersistenceIndexer.
• Using the default configuration, files larger than 4KB are stored on disk named after their SHA1 digest.  So these files are already on disk, and can be matched with their associated metadata records using the SHA1.

Requirements

1. F4 RDF Fedora 4 resources shall be persisted to disk as exploded BagIt bags, in a directory tree distinct separate from the repository's primary storage
2. F4 NonRDF resources shall be optionally persisted to disk in a directory tree distinct from the repository primary storage
3. The directory structure of the Bags shall have a discoverable and predictable relationship with the resource's repository URL
4. F4 RDF resources shall be persisted to disk in a client-defined RDF serialization, from the following options: application/ld+json, text/rdf+n3, application/rdf+xml, or text/turtle
5. The directory structure of the persisted NonRDF resources shall have logic relationship with the resource's repository path
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1. be associated with their respective RDF resources by the following optional modes:
   
   1. copying the NonRDF resource to the Bag's data directory
   2. hard-linking from the Bag's data directory to the NonRDF resource in the repository's primary storage (requires the Bag and repository storage to be on the same filesystem)
   3. sym-linking from the Bag's data directory to the NonRDF resource in the repository's primary storage 
   4. creating a manifest with NonRDF repository URLs (holey bags)