Lisp - write to file using low memory footprint

Question

I have large hash tables that I am writing to disk as an occasional backup. I am finding that as I map the hash tables and write to a file, the RAM usage skyrockets compared to

Answer 1

Try loop to loop over the hash-tables.

sth like:

(loop for k1 
      being the hash-key 
      using (hash-value v1) of (customer-var1 cust-data)
      do (format s "~A ~A~%" k1 v1))

Or if you don't need the values:

(loop for k being the hash-key of (customer-var2 cust-data)
      do (format ))

Originally I thought maphash would collect values but it does not, as @tfb pointed out. Then I don't know.