[libvirt] [PATCH] init: raise default system aio limits

[Eric Blake]

https://bugzilla.redhat.com/show_bug.cgi?id=740899 documents that
if qemu uses aio=native for its disks, then it consumes 128 aio
requests per disk.  On a host with multiple guests, this can quickly
run out of kernel aio requests with the default aio-max-nr of
65536.  Kernel developers have confirmed that there is no up-front
cost to raising this limit (a larger limit merely implies that more
aio requests can be issued in parallel, which in turn will result
in more kernel memory allocation if the system really does use that
many requests).  Since the system default limit prevents 256 disks,
which is well within libvirt's current scalability, this patch
installs a file to raise the limit and document it in case a system
administrator has further cause to tune the limit.  The install
only works on platforms new enough to source /etc/sysctl.d/*
alongside /etc/sysctl.conf (F14 and RHEL 6).

* daemon/libvirtd.sysctl: New file.
* daemon/Makefile.am (EXTRA_DIST): Ship it.
(install-init, uninstall-init): Install it.
* libvirt.spec.in (%files): Include it in rpm.
---

So far, I've confirmed that this passes 'make distcheck' (well, once
https://www.redhat.com/archives/libvir-list/2011-October/msg00074.html
is applied) and 'make rpm', although I didn't actually try installing
the resulting rpm nor rebooting a system to see if it takes effect.

 daemon/Makefile.am     |   11 ++++++++---
 daemon/libvirtd.sysctl |    8 ++++++++
 libvirt.spec.in        |    5 +++++
 3 files changed, 21 insertions(+), 3 deletions(-)
 create mode 100644 daemon/libvirtd.sysctl

diff --git a/daemon/Makefile.am b/daemon/Makefile.am
index 690bf85..1cf2b73 100644
--- a/daemon/Makefile.am
+++ b/daemon/Makefile.am
@@ -38,6 +38,7 @@ EXTRA_DIST =						\
 	libvirtd.policy-1				\
 	libvirtd.sasl					\
 	libvirtd.sysconf				\
+	libvirtd.sysctl					\
 	libvirtd.aug                                    \
 	libvirtd.logrotate.in                           \
 	libvirtd.qemu.logrotate.in                      \
@@ -252,16 +253,20 @@ install-logrotate: $(LOGROTATE_CONFS)

 if LIBVIRT_INIT_SCRIPT_RED_HAT
 install-init: libvirtd.init
-	mkdir -p $(DESTDIR)$(sysconfdir)/rc.d/init.d
+	mkdir -p $(DESTDIR)$(sysconfdir)/rc.d/init.d \
+	  $(DESTDIR)$(sysconfdir)/sysconfig \
+	  $(DESTDIR)$(sysconfdir)/sysctl.d
 	$(INSTALL_SCRIPT) libvirtd.init \
 	  $(DESTDIR)$(sysconfdir)/rc.d/init.d/libvirtd
-	mkdir -p $(DESTDIR)$(sysconfdir)/sysconfig
 	$(INSTALL_DATA) $(srcdir)/libvirtd.sysconf \
 	  $(DESTDIR)$(sysconfdir)/sysconfig/libvirtd
+	$(INSTALL_DATA) $(srcdir)/libvirtd.sysctl \
+	  $(DESTDIR)$(sysconfdir)/sysctl.d/libvirtd

 uninstall-init:
 	rm -f $(DESTDIR)$(sysconfdir)/rc.d/init.d/libvirtd \
-		$(DESTDIR)$(sysconfdir)/sysconfig/libvirtd
+		$(DESTDIR)$(sysconfdir)/sysconfig/libvirtd \
+		$(DESTDIR)$(sysconfdir)/sysctl.d/libvirtd

 BUILT_SOURCES += libvirtd.init

diff --git a/daemon/libvirtd.sysctl b/daemon/libvirtd.sysctl
new file mode 100644
index 0000000..3c70884
--- /dev/null
+++ b/daemon/libvirtd.sysctl
@@ -0,0 +1,8 @@
+# The kernel allocates aio memory on demand, and this number limits the
+# number of parallel aio requests; the only drawback of a larger limit is
+# that a malicious guest could issue parallel requests to cause the kernel
+# to set aside memory.  Set this number at least as large as
+#   128 * (number of virtual disks on the host)
+# Libvirt uses a default of 1M requests to allow 8k disks, with at most
+# 64M of kernel memory if all disks hit an aio request at the same time.
+fs.aio-max-size = 1048576
diff --git a/libvirt.spec.in b/libvirt.spec.in
index b87e3f6..9f5797a 100644
--- a/libvirt.spec.in
+++ b/libvirt.spec.in
@@ -962,6 +962,11 @@ fi
 %doc daemon/libvirtd.upstart
 %config(noreplace) %{_sysconfdir}/sysconfig/libvirtd
 %config(noreplace) %{_sysconfdir}/libvirt/libvirtd.conf
+%if 0%{?fedora} >= 14 || 0%{?rhel} >= 6
+%config(noreplace) %{_sysconfdir}/sysctl.d/libvirtd
+%else
+rm -f $RPM_BUILD_ROOT%{_sysconfdir}/sysctl.d/libvirtd
+%endif
 %if %{with_dtrace}
 %{_datadir}/systemtap/tapset/libvirtd.stp
 %endif
-- 
1.7.4.4