MongoDB: What cause bytesRead in insert operation?

Recently we had high CPU/Memory and I/O usage on our MongoDB. While checking the logs all I found is some insert during this period. While inspecting logs I noticed most of the insert logs have bytesRead in the storage section. So I suspect this cause I/O then caching the data cause high memory. After the insert spike the I/O and CPU went down but memory stayed the same which after a restart got resolved. Is this disk read normal with insert operation? We are using **Mongo v4.0** with WiredTiger storage engine in CentOS7 VM. 2024-02-14T23:39:44.533+0800 I COMMAND [conn939845] insert db.user_log ninserted:1 keysInserted:11 numYields:0 locks:{ Global: { acquireCount: { r: 1, w: 1 } }, Database: { acquireCount: { w: 1 } }, Collection: { acquireCount: { w: 1 } } } storage:{ data: { bytesRead: 34390, timeReadingMicros: 140837 } } 141ms 2024-02-14T23:40:16.785+0800 I COMMAND [conn939845] insert db.user_log ninserted:1 keysInserted:11 numYields:0 locks:{ Global: { acquireCount: { r: 1, w: 1 } }, Database: { acquireCount: { w: 1 } }, Collection: { acquireCount: { w: 1 } } } storage:{ data: { bytesRead: 24150, timeReadingMicros: 506594 } } 507ms