At a glance
Memory Resident (MB) is the amount of physical RAM the mongod process currently has resident, read live from the server. It is a capacity signal: it shows how much real memory the database is holding, which over time tells you whether the working set still fits in RAM or is pushing against the host’s limit. Read it as a trend against your host’s total memory rather than as a pass/fail number.
| What it tracks | Resident physical memory held by the mongod process, in megabytes. |
| Data source | mem.resident from serverStatus. |
| Time window | RT (real-time, refreshed every 60 seconds). |
| Alert trigger | None by default. This is a trend and context card; pair it with the WiredTiger cache and disk cards for the alerting layer. |
| Roles | owner, platform, sre, dba |
What it tracks
The card reportsserverStatus.mem.resident, the resident-set size of the mongod process expressed in megabytes. This is the real physical memory the process holds right now, distinct from virtual memory (mem.virtual) and from the WiredTiger storage-engine cache, which is a configured subset of this footprint. On the WiredTiger storage engine, resident memory tends to grow toward a steady plateau as the cache fills with the working set, then stays roughly flat: that plateau is healthy. The number is most useful as a trend read against the host’s total RAM. Resident memory sitting close to physical RAM, especially alongside a falling WiredTiger Cache Hit Rate %, is the early sign that the working set no longer fits in memory and reads are spilling to disk. A single instantaneous value means little; the shape over hours and days is what matters.
Reconciling against the source
Confirm the figure in MongoDB’s own tooling withdb.serverStatus().mem.resident in mongosh, or watch the res column in mongostat. On Atlas, the Metrics tab exposes a System Memory / process-memory chart for the same series. Expect small differences from operating-system tools such as top or ps (which may report the resident set at a slightly different instant or rounding), and remember the value is per-mongod, normally the primary.