Overcommitting With Kvm - Red Hat ENTERPRISE LINUX 5 - VIRTUALIZATION GUIDE Manual

getting image information
the info parameter displays information about a disk image. the format for the info option is as
follows:
# qemu-img info [-f format] filename
give information about the disk image filename. use it in particular to know the size reserved on disk
which can be different from the displayed size. if vm snapshots are stored in the disk image, they are
displayed too.
Supported formats
The format of an image is usually guessed automatically. The following formats are supported:
raw
Raw disk image format (default). This format has the advantage of being simple and easily
exportable to all other emulators. If your file system supports holes (for example in ext2 or ext3
on Linux or NTFS on Windows), then only the written sectors will reserve space. Use qemu-img
info to know the real size used by the image or ls -ls on Unix/Linux.
qcow2
QEMU image format, the most versatile format. Use it to have smaller images (useful if your file
system does not supports holes, for example: on Windows), optional AES encryption, zlib based
compression and support of multiple VM snapshots.
qcow
Old QEMU image format. Only included for compatibility with older versions.
cow
User Mode Linux Copy On Write image format. The cow format is included only for compatibility
with previous versions. It does not work with Windows.
vmdk
VMware 3 and 4 compatible image format.
cloop
Linux Compressed Loop image, useful only to reuse directly compressed CD-ROM images
present for example in the Knoppix CD-ROMs.

31.4. Overcommitting with KVM

The KVM hypervisor supports overcommitting CPUs and overcommitting memory. Overcommitting is
allocating more virtualized CPUs or memory than there are physical resources on the system. With
CPU overcommit, under-utilized virtualized servers or desktops can run on fewer servers which saves
power and money.
Xen support
CPU overcommitting is not supported for the Xen hypervisor. Overcommitting CPUs with
the Xen hypervisor may cause system instability and crashes of the host and virtualized
guests.
Overcommitting with KVM
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