Fast Virtio VM

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This page explains a way to build a blazing fast Gentoo VM under KVM using Virtio and mdev.

Steps

Use a fast computer

No really, this is probably the most important thing of all, don't expect a 2 second booting VM if you are on a P3 with a slow IDE disk. This tutorial is Build on a CentOS 6.2 KVM HypverVisor on a Dual QuadCore Xeon 2.4Ghz with 8 (v)CPU's and 2GB RAM assigned to the VM. The disk file is on an SSD (old Intel x25-M 80GB, so there is more to gain if you use a faster SSD).

Use ext2

It has the fastest mount time.

Kernel

Compile the kernel with:

VirtIO support
Hugetlb support

NO usb support (Unless planning to passthrough USB from the host).
NO HID support (see above).
NO extended video/device drivers/filesystem drivers that will not be used. (duh)
NO fuse support (Slow...)
NO modules (Slower...)

Kernel boot options

noapic (APIC is for hardware management, and since we are in VM and the hardware is managed by our host, we don't need it).
notsc (TSC is CPU clock synchronization, not needed in a VM).

Because they cannot be removed from the kernel itself, no need these as kernel boot options.

Use a static IP

DHCP is slow.

Use mdev

Replace udev with mdev (BusyBox), udev is dead slow vs mdev.

Configure mdev

Remove useless mdev devices (tun,tap, all sound, hd*, sd*, fd*, md*, grsec(*), zap, dvb, v4l stuff)

Prelink

Prelink all binaries:

root #emerge --ask prelink

root #prelink -amRf

OpenRC

Use rc-update to delete all non-essential daemons (git, consolefont, devfs, keymaps, modules, sysfs, swclock, staticroute)

tmpfs

Use tmpfs for /tmp and /var/run (will not necessarily speedup boot, but for startup services that /tmp heavily, it might help).

Results

I created my own boot time init script, which echoes the current kernel uptime.

Please keep in mind that all of this is without *any* dirty tricks like initrd or initramfs. It's just static sequential boot. Even using parallel start of services (thru RC_PARALLEL="yes") would slow down the system.