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More KVM Modules Configuration

Last year I blogged about blacklisting a video driver so that KVM virtual machines didn’t go into graphics mode [1]. Now I’ve been working on some other things to make virtual machines run better.

I use the same initramfs for the physical hardware as for the virtual machines. So I need to remove modules that are needed for booting the physical hardware from the VMs as well as other modules that get dragged in by systemd and other things. One significant saving from this is that I use BTRFS for the physical machine and the BTRFS driver takes 1M of RAM!

The first thing I did to reduce the number of modules was to edit /etc/initramfs-tools/initramfs.conf and change “MODULES=most” to “MODULES=dep”. This significantly reduced the number of modules loaded and also stopped the initramfs from probing for a non-existant floppy drive which added about 20 seconds to the boot. Note that this will result in your initramfs not supporting different hardware. So if you plan to take a hard drive out of your desktop PC and install it in another PC this could be bad for you, but for servers it’s OK as that sort of upgrade is uncommon for servers and only done with some planning (such as creating an initramfs just for the migration).

I put the following rmmod commands in /etc/rc.local to remove modules that are automatically loaded:
rmmod btrfs
rmmod evdev
rmmod lrw
rmmod glue_helper
rmmod ablk_helper
rmmod aes_x86_64
rmmod ecb
rmmod xor
rmmod raid6_pq
rmmod cryptd
rmmod gf128mul
rmmod ata_generic
rmmod ata_piix
rmmod i2c_piix4
rmmod libata
rmmod scsi_mod

In /etc/modprobe.d/blacklist.conf I have the following lines to stop drivers being loaded. The first line is to stop the video mode being set and the rest are just to save space. One thing that inspired me to do this is that the parallel port driver gave a kernel error when it loaded and tried to access non-existant hardware.
blacklist bochs_drm
blacklist joydev
blacklist ppdev
blacklist sg
blacklist psmouse
blacklist pcspkr
blacklist sr_mod
blacklist acpi_cpufreq
blacklist cdrom
blacklist tpm
blacklist tpm_tis
blacklist floppy
blacklist parport_pc
blacklist serio_raw
blacklist button

On the physical machine I have the following in /etc/modprobe.d/blacklist.conf. Most of this is to prevent loading of filesystem drivers when making an initramfs. I do this because I know there’s never going to be any need for CDs, parallel devices, graphics, or strange block devices in a server room. I wouldn’t do any of this for a desktop workstation or laptop.
blacklist ppdev
blacklist parport_pc
blacklist cdrom
blacklist sr_mod
blacklist nouveau

blacklist ufs
blacklist qnx4
blacklist hfsplus
blacklist hfs
blacklist minix
blacklist ntfs
blacklist jfs
blacklist xfs

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