pageserver: increase DEFAULT_MAX_WALRECEIVER_LSN_WAL_LAG (#6970)

## Problem

At high ingest rates, pageservers spuriously disconnect from safekeepers
because stats updates don't come in frequently enough to keep the
broker/safekeeper LSN delta under the wal lag limit.

## Summary of changes

- Increase DEFAULT_MAX_WALRECEIVER_LSN_WAL_LAG from 10MiB to 1GiB. This
should be enough for realistic per-timeline throughputs.

pageserver/src/tenant/config.rs

@@ -52,7 +52,10 @@ pub mod defaults {
     pub const DEFAULT_PITR_INTERVAL: &str = "7 days";
     pub const DEFAULT_WALRECEIVER_CONNECT_TIMEOUT: &str = "10 seconds";
     pub const DEFAULT_WALRECEIVER_LAGGING_WAL_TIMEOUT: &str = "10 seconds";
-    pub const DEFAULT_MAX_WALRECEIVER_LSN_WAL_LAG: u64 = 10 * 1024 * 1024;
+    // The default limit on WAL lag should be set to avoid causing disconnects under high throughput
+    // scenarios: since the broker stats are updated ~1/s, a value of 1GiB should be sufficient for
+    // throughputs up to 1GiB/s per timeline.
+    pub const DEFAULT_MAX_WALRECEIVER_LSN_WAL_LAG: u64 = 1024 * 1024 * 1024;
     pub const DEFAULT_EVICTIONS_LOW_RESIDENCE_DURATION_METRIC_THRESHOLD: &str = "24 hour";
 
     pub const DEFAULT_INGEST_BATCH_SIZE: u64 = 100;

test_runner/regress/test_tenant_conf.py

@@ -270,7 +270,7 @@ eviction_policy = { "kind" = "LayerAccessThreshold", period = "20s", threshold =
         "period": "20s",
         "threshold": "23h",
     }
-    assert final_effective_config["max_lsn_wal_lag"] == 10 * 1024 * 1024
+    assert final_effective_config["max_lsn_wal_lag"] == 1024 * 1024 * 1024
 
     # restart the pageserver and ensure that the config is still correct
     env.pageserver.stop()