Tune libvirtd connection limits for OpenStack

The libvirt daemon ships with max_workers=20 and max_client_requests=5.
Those defaults suit a multi-tenant host serving many mutually
distrustful clients, where the per-client request cap protects the
daemon from any single client monopolising it. Under Kolla-Ansible,
nova-compute is effectively the sole privileged client of libvirtd, so
the cap of 5 concurrent requests per client becomes the binding
constraint rather than a safety net.

When several instances build at once -- as they do during the tempest
run of the upgrade jobs -- nova-compute's concurrent libvirt RPCs
(instance defineXML calls plus the resource-tracker periodic) exceed
five. Further requests queue, the connection's keepalive responses are
starved, and after keepalive_interval (5s) x keepalive_count (5) = 25s
libvirtd closes the connection. nova then sees "Cannot recv data:
Input/output error" mid-defineXML, the build is rescheduled, and once
all hosts are exhausted the instance is left in ERROR. This presents as
flaky upgrade-job failures.

Example failure, on the voting kolla-ansible-debian-bookworm-upgrade
job (stable/2025.2):

  https://zuul.opendev.org/t/openstack/build/73ff5a97567d4e05b1774170e0f4aada

with primary/logs/kolla/libvirt/libvirtd.txt showing libvirt's own
recommendation to tune this parameter:

  warning : Client hit max requests limit 5. This may result in
  keep-alive timeouts. Consider tuning the max_client_requests server
  parameter
  ...
  error : connection closed due to keepalive timeout

Raise the per-client cap to 20 -- comfortably above the handful of
concurrent builds plus periodics seen in practice -- and the worker
pool to 50 so that lifting the per-client limit does not starve other
connections such as live migration, health checks and periodic tasks.
Both values are exposed as variables (libvirt_max_client_requests and
libvirt_max_workers) so operators can tune them further.

This is not a Kolla-specific workaround -- the stock libvirtd.conf
comment warns that setting max_client_requests too low "may cause
keepalive timeouts", and since 2022 libvirtd emits the runtime warning
quoted above. As Daniel Berrange explained when he added that warning,
the keepalive packets "are subject to the 'max_client_requests' limit
just like any API calls", so a client that saturates the limit can have
its connection dropped:

  https://lists.libvirt.org/archives/list/devel@lists.libvirt.org/message/5NIJPA7SJVY55VSK7RTEDEHKWOZYJKT6/

Red Hat documents the same tuning for libvirt hosts under load in its
knowledge base, notably "Too many active clients are dropping the
connection" at https://access.redhat.com/solutions/28146.

Assisted-By: Claude Opus 4.8 (1M context, medium effort)
Partial-Bug: 2158515
Change-Id: I131c3d4d57dc73ca4dd421cc8f28465248c9830e
Signed-off-by: Michael Still <mikal@stillhq.com>

Author: mikalstill

ansible/roles/nova-cell/defaults/main.yml

@@ -639,6 +639,17 @@ libvirt_sasl_authname: "nova"
 libvirt_sasl_mech_list:
   - "{{ 'SCRAM-SHA-256' if libvirt_tls | bool else 'DIGEST-MD5' }}"
 
+# Connection processing limits for libvirtd. nova-compute is effectively the
+# only privileged client of libvirtd under Kolla, so libvirt's stock per-client
+# request cap (max_client_requests=5) is the binding constraint rather than a
+# safety net: concurrent instance builds plus the resource-tracker periodic
+# routinely exceed five in-flight RPCs, which starves keepalive responses until
+# libvirtd drops the connection. Raise the per-client cap, and the worker pool
+# along with it so that lifting the per-client limit does not starve other
+# connections (live migration, health checks, periodics).
+libvirt_max_client_requests: 20
+libvirt_max_workers: 50
+
 ####################
 # Kolla
 ####################

ansible/roles/nova-cell/templates/libvirtd.conf.j2

@@ -25,3 +25,13 @@ listen_addr = "{{ migration_interface_address }}"
 # Enable read-only access to libvirt socket
 auth_unix_ro = "none"
 {% endif %}
+
+# Connection processing limits. libvirt's defaults (max_workers=20,
+# max_client_requests=5) assume a multi-tenant host with many mutually
+# distrustful clients. Under Kolla, nova-compute is effectively the sole
+# privileged client, so the per-client cap of 5 becomes the binding
+# constraint: concurrent builds plus the resource-tracker periodic exceed
+# five in-flight RPCs and starve keepalive responses, causing libvirtd to
+# drop the connection after keepalive_interval (5s) x keepalive_count (5).
+max_workers = {{ libvirt_max_workers }}
+max_client_requests = {{ libvirt_max_client_requests }}

doc/source/reference/compute/libvirt-guide.rst

@@ -198,3 +198,26 @@ Since the Yoga release, the ``kolla-ansible certificates`` command generates
 certificates for libvirt TLS. A single key and certificate is used for all
 hosts, with a Subject Alternative Name (SAN) entry for each compute host
 hostname.
+
+Libvirt Connection Limits
+=========================
+
+By default ``libvirt`` is configured by upstream packages with reasonable
+defaults for an environment where many users are starting and stopping virtual
+machines. This isn't true for Kolla Hypervisor nodes, where a single
+``nova-compute`` container manages all of the virtual machine instances on the
+machine.
+
+Kolla-Ansible therefore tunes the following values in ``libvirtd.conf``:
+
+* libvirt_max_client_requests: raised to 20 compared to the usual default of
+  5.
+* libvirt_max_workers: raised to 50 compared to the usual default of 20.
+
+Both of these values may be overridden in ``globals.yml``.
+
+This means that ``nova-compute`` can therefore perform 20 operations via
+libvirt simultaneously instead of the usual default of 5. These values should
+be safe, but if you see libvirt error messages saying ``Client hit max requests
+limit``, you may want to consider an additional increase -- especially if you
+have particularly large hypervisors.

releasenotes/notes/libvirtd-connection-tuning-7c4f1e9a2b8d6f30.yaml

@@ -0,0 +1,18 @@
+---
+features:
+  - |
+    The ``nova_libvirt`` container's ``libvirtd.conf`` now raises libvirt's
+    connection processing limits above their stock defaults, which are tuned
+    for a multi-tenant host rather than for Kolla's single-privileged-client
+    pattern. ``max_client_requests`` is raised from 5 to 20 and ``max_workers``
+    from 20 to 50. Both are exposed as the ``libvirt_max_client_requests`` and
+    ``libvirt_max_workers`` variables so they can be tuned further.
+fixes:
+  - |
+    Fixed intermittent instance build failures (``Cannot recv data:
+    Input/output error`` during ``defineXML``, leading to instances in
+    ``ERROR`` state) that occurred when several instances were built
+    concurrently. nova-compute is effectively the sole client of ``libvirtd``,
+    and libvirt's default per-client request cap of 5 was being exceeded by
+    concurrent builds plus periodic tasks, starving keepalive responses until
+    ``libvirtd`` dropped the connection.