Belkin XM Commander Manuel d'utilisateur télécharger pdf (Page 28)

read-write sharing, export the directory to other domains via NFS from

domain0 (or use a cluster ﬁle system such as GFS or ocfs2).

5.2 Using File-backed VBDs

It is also possible to use a ﬁle in Domain 0 as the primary storage for a virtual machine.

As well as being convenient, this also has the advantage that the virtual block device

will be sparse — space will only really be allocated as parts of the ﬁle are used. So

if a virtual machine uses only half of its disk space then the ﬁle really takes up half of

the size allocated.

For example, to create a 2GB sparse ﬁle-backed virtual block device (actually only

consumes 1KB of disk):

# dd if=/dev/zero of=vm1disk bs=1k seek=2048k count=1

Make a ﬁle system in the disk ﬁle:

# mkfs -t ext3 vm1disk

(when the tool asks for conﬁrmation, answer ‘y’)

Populate the ﬁle system e.g. by copying from the current root:

# mount -o loop vm1disk /mnt

# cp -ax /{root,dev,var,etc,usr,bin,sbin,lib} /mnt

# mkdir /mnt/{proc,sys,home,tmp}

Tailor the ﬁle system by editing /etc/fstab, /etc/hostname, etc (don’t forget

to edit the ﬁles in the mounted ﬁle system, instead of your domain 0 ﬁlesystem, e.g.

you would edit /mnt/etc/fstab instead of /etc/fstab ). For this example put

/dev/sda1 to root in fstab.

Now unmount (this is important!):

# umount /mnt

In the conﬁguration ﬁle set:

disk = [’file:/full/path/to/vm1disk,sda1,w’]

As the virtual machine writes to its ‘disk’, the sparse ﬁle will be ﬁlled in and consume

more space up to the original 2GB.

Note that ﬁle-backed VBDs may not be appropriate for backing I/O-intensive

domains. File-backed VBDs are known to experience substantial slowdowns under

heavy I/O workloads, due to the I/O handling by the loopback block device used to

support ﬁle-backed VBDs in dom0. Better I/O performance can be achieved by using

either LVM-backed VBDs (Section 5.3) or physical devices as VBDs (Section 5.1).

Linux supports a maximum of eight ﬁle-backed VBDs across all domains by default.

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Belkin XM Commander Manuel d'utilisateur Page 28