For big deployments where mongodump/mongorestore approach is not feasible and backups are taken at FS level, we need way to restore individual databases and/or collections to the original system and to some other, new system. This feature is critical for success of those users who have multi-TB multi-tenant clusters and only want to restore a part of it.

With MMAP you could do the per-db backup/restore when directoryPerDb is used, but we no longer have the ability to use that approach in WT as WT metadata files are global for all namespaces. It will segfault if some *.wt files are not present.

However, with WT we have the advantage of individual files per namespace, so it should be possible to extract them and their metadata from an existing dbpath. Perhaps we will need a separate tool for that.