For some backend types, your slapd(8) should not be running (at least, not in read-write mode) when you do this to ensure consistency of the database. It is always safe to run slapcat with the slapd-bdb(5), slapd-hdb(5), and slapd-null(5) backends.
so to answer one needs to know what backend you're referring to.
Ok. I use bdb backend, so I have to use slapd-bdb(5) as reference. I've not found any word about the possibility or not to make copies "on the fly".
Please carefully re-read the above text: "IT IS ALWAYS SAFE TO RUN SLAPCAT WITH THE SLAPD-BDB(5) BACKEND".
p.