libvirt/daemon/libvirtd.sysctl

# 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-nr = 1048576
init: raise default system aio limits 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, only 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. 2011-10-04 15:44:25 +00:00			`# 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-nr = 1048576`