On a brief background read it looks like another issue is that blobs are not necessarily stored in contiguous regions of the file. Maybe there's some clever way to combine a VFS shim and virtual tables to store page aligned blobs in a region of the same file as the main database.
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Exactly, a guaranteed alignment and continuity of a blob makes it memory-mappable allowing to avoid copying the data back and forth.
On a brief background read it looks like another issue is that blobs are not necessarily stored in contiguous regions of the file. Maybe there's some clever way to combine a VFS shim and virtual tables to store page aligned blobs in a region of the same file as the main database.