Nova:libvirt the life cycle of image

Translated from: http://www.pixelbeat.org/docs/openstack_libvirt_images/

The main stages of a Virtual machine disk image as it transfers through OpenStack to be booted under Libvirt is:

Initially the image is downloaded from glance and cached in Libvirt base. We'll consider the options for handling a qcow2 image stored in glance, as/format can be downloaded quite efficiently From glance as it supports compression, and image sparseness can maintained. This article'll focus on the flow and transformations in "Libvirt base", which are used to cache, preprocess and optional Ly back, VM disk images.

Configuration

First we ' ll summarize the config variables involved, before presenting the operations associated with each config Combinat Ion, in each OpenStack release. Note I ' m describing upstream openstack here, and not my employer ' s Red Hat OpenStack which have back-ported enhancements be Tween versions where appropriate.

Config	Default	Release	Description
Use_cow_images	True	Cactus	Whether to use CoW images for "libvirt instance disks"
Force_raw_images	True	Essex	Allows disabling convert to Raw in "libvirt base" for operational reasons
Libvirt_images_type	' Default '	Folsom	Deprecates use_cow_images and allows selecting LVM libvirt images
[Libvirt]/images_type	' Default '	Icehouse	Deprecates Libvirt_images_type in the [DEFAULT] section
Preallocate_images	' None '	Grizzly	Instance disks preallocation mode. ' Space ' = fallocate images
Resize_fs_using_block_device	False	Havana	Allows enabling of direct resize for qcow2 images

The main reason that raw images was written in ' Libvirt base ' by default (since Diablo), was to remove possible compression From the Qcow2 image received from glance. Note compression in Qcow2 images are read only, and so this would impact reads from unwritten portions of the qcow2 image. Users may want to the change this option, depending on CPU resources and I/O bandwidth available. For example, systems with slower I/O or less space available, could want to trade the higher CPU requirements of compression , to minimize input bandwidth. Note RAW images is used unconditionally with LIBVIRT_IMAGES_TYPE=LVM.

Whether to use CoW images for the "Libvirt instance disks" also depends on I/O characteristics of the user ' s system. Without CoW, more space would be used to common parts of the disk image, but on the flip side depending on the backing Sto Re and host caching, there may is better concurrency achieved by have each VM operate on its own copy.

Enabling preallocation of space for the ' Libvirt instance disks ' can help with both space guarantees and I/O performance. Even when isn't using CoW instance disks, the copy each VM gets be sparse and so the VM could fail unexpectedly at run time WI Th ENOSPC. By running Fallocate (1) on the instance disk images, we immediately and efficiently allocate the space for them in the FIL e system (if supported). Also run time performance should be improved as the file system doesn ' t has to dynamically allocate blocks at run time, R Educing CPU Overhead and more importantly file fragmentation.

Disk Image Operations

For each release and config combination, here is the created files and associated operations in getting a qcow2 Image from glance through to being booted on a libvirt Virtual machine.

Folsom, Force_raw_images=true, use_cow_images=true

This results is instance booting from a CoW image, and backed by a resized raw image.

Nova command	Source Code	Notes
wget http://glance/$image-o base_/$hex. part	Images.fetch
Qemu-img Convert-o Raw $hex. Part $hex. Converted	Images.fetch_to_raw	Creates sparse file
MV $hex. Converted $hex; RM $hex. Part	Images.fetch_to_raw
	Imagebackend.create_image
CP $hex $hex _$size	Libvirt.utils.copy_image	Creates sparse file
Qemu-img Resize $hex _$size $size	Disk.extend
RESIZE2FS $hex _size	Disk.extend	unpartitioned ext[234]
Qemu-img create-f qcow2-o backing_file= ... $instance _dir/disk	Libvirt.utils.create_image

Folsom, Force_raw_images=true, Use_cow_images=false

This results is instance booting from a copy of a resized raw image.

Nova command	Source code	Notes
wget http://glance/$image-o  base_ /$hex. Part	images.fetch	&n BSP;
qemu-img convert-o Raw $hex. Part $hex. Converted	Images.fetch_to_raw	creates spars E file
mv $hex. Converted $hex;  rm $hex. Part	images.fetch_to_raw	< /td>
	imagebackend.create_image
CP $hex &NBSP ; $instance _dir /disk	libvirt.utils.copy_image
qemu-img Resize Disk	disk.extend
resize2fs disk	disk.extend	unpartitioned ext[234]

Folsom, Force_raw_images=false, Use_cow_images=false

This results is instance booting from a copy of a resized qcow2 image.

Nova command	Source Code	Notes
wget http://glance/$image-o base_/$hex. part	Images.fetch
MV $hex. Part $hex	Images.fetch_to_raw
	Imagebackend.create_image
CP $hex $instance _dir/disk	Libvirt.utils.copy_image
QEMU-IMG Resize Disk	Disk.extend
RESIZE2FS disk	Disk.extend	Ignored for Qcow2¹

Folsom, Force_raw_images=false, use_cow_images=true

This results the instance booting from a CoW image, and backed by a resized qcow2 image.

Nova command	Source code	Notes
wget http://glance/$image-o  base_ /$hex. Part	images.fetch	&n BSP;
mv $hex. Part $hex	images.fetch_to_raw
	Imagebackend.create_image
cp $hex $hex _$size	libvirt.utils.copy_image
qemu-img Resize $hex _$size $size	disk.extend
resize2fs $hex _size	disk.extend	ignored for qcow2 ¹
qemu-img create-f qcow2-o backing_file=...  $instance _dir /disk	l Ibvirt.utils.create_image

Grizzly, Force_raw_images=true, use_cow_images=true

Grizzly introduces a change for use_cow_images=True, where it'll resize in the $instance _dir rather than in BAS E_. So the resize are not being cached, but that's minimal CPU tradeoff per instance boot, for the extra space saved in Base_. We ' ll just present the default Config values here which illustrates the only significant change from Folsom.
This results is instance booting from a resized CoW image, and backed by a raw image.

Nova command	Source Code	Notes
wget http://glance/$image-o base_/$hex. part	Images.fetch
Qemu-img Convert-o Raw $hex. Part $hex. Converted	Images.fetch_to_raw	Creates sparse file
MV $hex. Converted $hex; RM $hex. Part	Images.fetch_to_raw
	Imagebackend.create_image
Qemu-img create-f qcow2-o backing_file= ... $instance _dir/disk	Libvirt.utils.create_image
QEMU-IMG Resize Disk $size	Disk.extend
RESIZE2FS disk	Disk.extend	Grizzly always ignores²

[² Update SEP 2013:stanislaw Pitucha also noticed the above referenced Grizzly change introduced a regression where unpartitioned qcow2 images were no longer resized. See the Havana resize_fs_using_block_device option below for details.]

Grizzly, preallocate_images= ' space '

Grizzly also has a new fallocate functionality in this area controlled by the preallocate_images config option. If that was set to ' space ' and then after the operations above, the $instance _dir/images would be fallocated to immediately de Termine If enough space is available, and to possibly improve VM I/O performance due to ongoing allocation avoidance, and Better locality of block allocations.

¹havana, Resize_fs_using_block_device=false

As noted in the first Grizzly change above, Stanislaw Pitucha noticed ' change introduced a regression where unpartitio Ned Qcow2 images were no longer resized. He supplied a fix to resize qcow directly rather than relying on the raw image being available, which would also cater for The Force_raw_images=false case, even pre Grizzly did not. This new option can is used to enable the this support, but there is some large performance and possible security issues so I T ' s not enabled by default. This support would be available in the upcoming Havana release.

General performance considerations

Performance have improved in this area through each OpenStack release, with some of the main topics to consider, for past a nd future changes being:

Minimize I/O

Note these were implemented in Essex:

• Copy images, then resize, rather than vice versa
• Directly generating images in the $instance _dir/
• Intelligent reading of sparse input
• Reproduction of sparse input on output
• Use compression

Note this is implemented in Folsom:

• Avoid file sytem overhead by setting LIBVIRT_IMAGES_TYPE=LVM. Note file system overhead varies depending on file system

Minimize Storage

• Use compression
• Use sparse output/generation
• Avoid resized copies when not needed
• Use CoW if appropriate

Improve Caching

• Avoid thrashing the page cache with large intermediate images
• Improve low level caching through better storage allocation

Preprocessing

• Preprocessing May is possible on images like Preallocation=metadata which trades off initial CPU cost for possibly much is Tter run time I/O performance
• Such cost would is some what alleviated by have asynchronous population of the Base_ cache

Nova:libvirt the life cycle of image