On 10/04/2011 12:59 PM, Eric Blake wrote:
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).
ACK. This all makes sense and seems fairly thoroughly researched. (Would be good to actually try it, though)
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