Albert Barnes Commentary 1 Chronicles 9:22

"All these that were chosen to be porters in the thresholds were two hundred and twelve. These were reckoned by genealogy in their villages, whom David and Samuel the seer did ordain in their office of trust." — 1 Chronicles 9:22 (ASV)

The porters, like the singers (Nehemiah 12:29), lived for the most part in the villages around Jerusalem. They were the descendants of those originally selected for the work by David. David’s arrangements are here regarded as having had the sanction of Samuel—which would imply that he planned them in the lifetime of Saul, while he was still a fugitive and an outlaw.