Colossus metadata is stored in Bigtable: see https://cloud.google.com/blog/products/storage-data-transfer/a-peek-behind-colossus-googles-file-system https://clo

Colossus metadata is stored in Bigtable: see https://cloud.google.com/blog/products/storage-data-transfer/a-peek-behind-colossus-googles-file-system https://cloud.google.com/blog/products/storage-data-transfer... Spanner stores its data in Colossus, so there would be some bootstrapping issues to resolve to move it to Spanner over Bigtable. (Bigtable also has the bootstrap issues but has solved them already and there are additional difficulties due to some details that I probably am not at liberty to share) Spanner is used for metadata for many other very very large storage systems, though.