mapping together the physical volumes /dev/sda2, /dev/sdc1,
and /dev/sde5 to create logical volumes that form one manage-
able group.
In Figure 1, lvdata, lvhome, and lvvar represent logical volumes,
which contain the file system and thus can be used as mount points
for directories such as /var/ftp, /home, and /usr. Each logical
volume is split into chunks of data, called logical extents. Within
a volume group, the size of the logical extent is the same as that
of the physical extent; that is, a one-to-one mapping exists between
the logical extents and the physical extents.
Each physical extent has a unique identification number on
a physical volume but does not necessarily have a unique identi-
fication number on a logical volume because several different
physical volumes can constitute one logical volume. Therefore, the
logical extent identification numbers also identify the associated
physical extents. Whenever the storage area is accessed, the address
or the identification number of the logical extent is used to actu-
ally perform I/O on the physical storage.
The volume group descriptor area (VGDA) is stored at the
beginning of each physical volume and functions similarly to the
partition table for LVM. The
VGDA contains one volume group
descriptor, one physical volume
descriptor, one logical volume
descriptor, and several physical
extent descriptors. When the
system is booted up, the volume
group and logical volumes are
activated and the VGDA is loaded
into memory. The VGDA allows
LVM to identify where the logical
volumes are actually stored. The
one-to-one mapping of physical
to logical volumes is necessary to
access the physical location and
to perform I/O operations.
Conceptualizing LVM processes
LVM comprises lvm-mod, a kernel module (that is, a device driver)
under General Public License (GPL), and applications that use the
module to perform storage-related management processes. In a typi-
cal scenario, administrators use an LVM command such as
pvcreate
or vgscan to perform storage management functions. For example,
Figure 2 represents the execution flow of an LVM task to retrieve the
status of a particular storage device, as follows:
1. LVM commands invoke the LVM kernel module, lvm-mod, to
perform the operating system–level tasks that are required to
service the request.
2. The device driver executes the request on the hardware
storage device, and the LVM kernel module configures the
device dynamically through the /proc file system.
3. LVM dynamically obtains information on the storage devices
by using the /proc file system.
STORAGE ENVIRONMENT
POWER SOLUTIONS November 2003
112
(286 paginas)
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