diff --git a/test_runner/batch_others/test_wal_acceptor.py b/test_runner/batch_others/test_wal_acceptor.py
index 8e33264a20..57cf379a96 100644
--- a/test_runner/batch_others/test_wal_acceptor.py
+++ b/test_runner/batch_others/test_wal_acceptor.py
@@ -3,13 +3,17 @@ import random
 import time
 import os
 import subprocess
+import sys
+import threading
 import uuid
 
 from contextlib import closing
+from dataclasses import dataclass, field
 from multiprocessing import Process, Value
 from fixtures.zenith_fixtures import PgBin, ZenithEnv, ZenithEnvBuilder
 from fixtures.utils import lsn_to_hex, mkdir_if_needed
 from fixtures.log_helper import log
+from typing import List, Optional
 
 pytest_plugins = ("fixtures.zenith_fixtures")
 
@@ -34,13 +38,22 @@ def test_normal_work(zenith_env_builder: ZenithEnvBuilder):
             assert cur.fetchone() == (5000050000, )
 
 
+@dataclass
+class BranchMetrics:
+    name: str
+    latest_valid_lsn: int
+    # One entry per each Safekeeper, order is the same
+    flush_lsns: List[int] = field(default_factory=list)
+    commit_lsns: List[int] = field(default_factory=list)
+
+
 # Run page server and multiple acceptors, and multiple compute nodes running
 # against different timelines.
 def test_many_timelines(zenith_env_builder: ZenithEnvBuilder):
     zenith_env_builder.num_safekeepers = 3
     env = zenith_env_builder.init()
 
-    n_timelines = 2
+    n_timelines = 3
 
     branches = ["test_wal_acceptors_many_timelines_{}".format(tlin) for tlin in range(n_timelines)]
 
@@ -50,21 +63,114 @@ def test_many_timelines(zenith_env_builder: ZenithEnvBuilder):
         env.zenith_cli(["branch", branch, "main"])
         pgs.append(env.postgres.create_start(branch))
 
+    tenant_id = uuid.UUID(env.initial_tenant)
+
+    def collect_metrics(message: str) -> List[BranchMetrics]:
+        with env.pageserver.http_client() as pageserver_http:
+            branch_details = [
+                pageserver_http.branch_detail(tenant_id=tenant_id, name=branch)
+                for branch in branches
+            ]
+        # All changes visible to pageserver (latest_valid_lsn) should be
+        # confirmed by safekeepers first. As we cannot atomically get
+        # state of both pageserver and safekeepers, we should start with
+        # pageserver. Looking at outdated data from pageserver is ok.
+        # Asking safekeepers first is not ok because new commits may arrive
+        # to both safekeepers and pageserver after we've already obtained
+        # safekeepers' state, it will look contradictory.
+        sk_metrics = [sk.http_client().get_metrics() for sk in env.safekeepers]
+
+        branch_metrics = []
+        with env.pageserver.http_client() as pageserver_http:
+            for branch_detail in branch_details:
+                timeline_id: str = branch_detail["timeline_id"]
+
+                m = BranchMetrics(
+                    name=branch_detail["name"],
+                    latest_valid_lsn=branch_detail["latest_valid_lsn"],
+                )
+                for sk_m in sk_metrics:
+                    m.flush_lsns.append(sk_m.flush_lsn_inexact[timeline_id])
+                    m.commit_lsns.append(sk_m.commit_lsn_inexact[timeline_id])
+
+                for flush_lsn, commit_lsn in zip(m.flush_lsns, m.commit_lsns):
+                    # Invariant. May be < when transaction is in progress.
+                    assert commit_lsn <= flush_lsn
+                # We only call collect_metrics() after a transaction is confirmed by
+                # the compute node, which only happens after a consensus of safekeepers
+                # has confirmed the transaction. We assume majority consensus here.
+                assert (2 * sum(m.latest_valid_lsn <= lsn
+                                for lsn in m.flush_lsns) > zenith_env_builder.num_safekeepers)
+                assert (2 * sum(m.latest_valid_lsn <= lsn
+                                for lsn in m.commit_lsns) > zenith_env_builder.num_safekeepers)
+                branch_metrics.append(m)
+        log.info(f"{message}: {branch_metrics}")
+        return branch_metrics
+
+    # TODO: https://github.com/zenithdb/zenith/issues/809
+    # collect_metrics("before CREATE TABLE")
+
     # Do everything in different loops to have actions on different timelines
     # interleaved.
     # create schema
     for pg in pgs:
         pg.safe_psql("CREATE TABLE t(key int primary key, value text)")
+    init_m = collect_metrics("after CREATE TABLE")
 
-    # Populate data
-    for pg in pgs:
+    # Populate data for 2/3 branches
+    class MetricsChecker(threading.Thread):
+        def __init__(self) -> None:
+            super().__init__(daemon=True)
+            self.should_stop = threading.Event()
+            self.exception: Optional[BaseException] = None
+
+        def run(self) -> None:
+            try:
+                while not self.should_stop.is_set():
+                    collect_metrics("during INSERT INTO")
+                    time.sleep(1)
+            except:
+                log.error("MetricsChecker's thread failed, the test will be failed on .stop() call",
+                          exc_info=True)
+                # We want to preserve traceback as well as the exception
+                exc_type, exc_value, exc_tb = sys.exc_info()
+                assert exc_type
+                e = exc_type(exc_value)
+                e.__traceback__ = exc_tb
+                self.exception = e
+
+        def stop(self) -> None:
+            self.should_stop.set()
+            self.join()
+            if self.exception:
+                raise self.exception
+
+    metrics_checker = MetricsChecker()
+    metrics_checker.start()
+
+    for pg in pgs[:-1]:
         pg.safe_psql("INSERT INTO t SELECT generate_series(1,100000), 'payload'")
 
-    # Check data
-    for pg in pgs:
+    metrics_checker.stop()
+
+    collect_metrics("after INSERT INTO")
+
+    # Check data for 2/3 branches
+    for pg in pgs[:-1]:
         res = pg.safe_psql("SELECT sum(key) FROM t")
         assert res[0] == (5000050000, )
 
+    final_m = collect_metrics("after SELECT")
+    # Assume that LSNs (a) behave similarly in all branches; and (b) INSERT INTO alters LSN significantly.
+    # Also assume that safekeepers will not be significantly out of sync in this test.
+    middle_lsn = (init_m[0].latest_valid_lsn + final_m[0].latest_valid_lsn) // 2
+    assert max(init_m[0].flush_lsns) < middle_lsn < min(final_m[0].flush_lsns)
+    assert max(init_m[0].commit_lsns) < middle_lsn < min(final_m[0].commit_lsns)
+    assert max(init_m[1].flush_lsns) < middle_lsn < min(final_m[1].flush_lsns)
+    assert max(init_m[1].commit_lsns) < middle_lsn < min(final_m[1].commit_lsns)
+    assert max(init_m[2].flush_lsns) <= min(final_m[2].flush_lsns) < middle_lsn
+    assert max(init_m[2].commit_lsns) <= min(final_m[2].commit_lsns) < middle_lsn
+
 
 # Check that dead minority doesn't prevent the commits: execute insert n_inserts
 # times, with fault_probability chance of getting a wal acceptor down or up
diff --git a/test_runner/fixtures/zenith_fixtures.py b/test_runner/fixtures/zenith_fixtures.py
index 0a32c5bffb..6330b28094 100644
--- a/test_runner/fixtures/zenith_fixtures.py
+++ b/test_runner/fixtures/zenith_fixtures.py
@@ -1,6 +1,6 @@
 from __future__ import annotations
 
-from dataclasses import dataclass
+from dataclasses import dataclass, field
 from cached_property import cached_property
 import asyncpg
 import os
@@ -11,6 +11,7 @@ import jwt
 import json
 import psycopg2
 import pytest
+import re
 import shutil
 import socket
 import subprocess
@@ -715,7 +716,7 @@ class ZenithPageserver(PgProtocol):
     def __exit__(self, exc_type, exc, tb):
         self.stop(True)
 
-    def http_client(self, auth_token: Optional[str] = None):
+    def http_client(self, auth_token: Optional[str] = None) -> ZenithPageserverHttpClient:
         return ZenithPageserverHttpClient(
             port=self.service_port.http,
             auth_token=auth_token,
@@ -1084,7 +1085,7 @@ class Safekeeper:
                 assert isinstance(res, dict)
                 return res
 
-    def http_client(self):
+    def http_client(self) -> SafekeeperHttpClient:
         return SafekeeperHttpClient(port=self.port.http)
 
 
@@ -1094,6 +1095,14 @@ class SafekeeperTimelineStatus:
     flush_lsn: str
 
 
+@dataclass
+class SafekeeperMetrics:
+    # These are metrics from Prometheus which uses float64 internally.
+    # As a consequence, values may differ from real original int64s.
+    flush_lsn_inexact: Dict[str, int] = field(default_factory=dict)
+    commit_lsn_inexact: Dict[str, int] = field(default_factory=dict)
+
+
 class SafekeeperHttpClient(requests.Session):
     def __init__(self, port: int) -> None:
         super().__init__()
@@ -1109,6 +1118,22 @@ class SafekeeperHttpClient(requests.Session):
         return SafekeeperTimelineStatus(acceptor_epoch=resj['acceptor_state']['epoch'],
                                         flush_lsn=resj['flush_lsn'])
 
+    def get_metrics(self) -> SafekeeperMetrics:
+        request_result = self.get(f"http://localhost:{self.port}/metrics")
+        request_result.raise_for_status()
+        all_metrics_text = request_result.text
+
+        metrics = SafekeeperMetrics()
+        for match in re.finditer(r'^safekeeper_flush_lsn{ztli="([0-9a-f]+)"} (\S+)$',
+                                 all_metrics_text,
+                                 re.MULTILINE):
+            metrics.flush_lsn_inexact[match.group(1)] = int(match.group(2))
+        for match in re.finditer(r'^safekeeper_commit_lsn{ztli="([0-9a-f]+)"} (\S+)$',
+                                 all_metrics_text,
+                                 re.MULTILINE):
+            metrics.commit_lsn_inexact[match.group(1)] = int(match.group(2))
+        return metrics
+
 
 def get_test_output_dir(request: Any) -> str:
     """ Compute the working directory for an individual test. """