2 - Raspberry PI Community Projects

e clustered into one VG and the slower ones into another; chunks of the first one can then be
assigned to applications requiring fast data access, while the second one will be kept for less
demanding tasks.
In any case, keep in mind that an LV isn't particularly attached to any one PV. It is possible to
influence where the data from an LV are physically stored, but this possibility isn't required
for day-to-day use. On the contrary: when the set of physical components of a VG evolves, the
physical storage locations corresponding to a particular LV can be migrated across disks (while
staying within the PVs assigned to the VG, of course).
12.1.2.2. Seing up LVM
Let us now follow, step by step, the process of setting up LVM for a typical use case: we want
to simplify a complex storage situation. Such a situation usually happens after some long and
convoluted history of accumulated temporary measures. For the purposes of illustration, we'll
consider a server where the storage needs have changed over time, ending up in a maze of
available partitions split over several partially used disks. In more concrete terms, the following
partitions are available:
• on the sdb disk, a sdb2 partition, 4 GB;
• on the sdc disk, a sdc3 partition, 3 GB;
• the sdd disk, 4 GB, in fully available;
• on the sdf disk, a sdf1 partition, 4 GB; and a sdf2 partition, 5 GB.
In addition, let's assume that disks sdb and sdf are faster than the other two.
Our goal is to set up three logical volumes for three different applications: a file server requiring
5 GB of storage space, a database (1 GB) and some space for back-ups (12 GB). The first two
need good performance, but back-ups are less critical in terms of access speed. All these constraints
prevent the use of partitions on their own; using LVM can abstract the physical size of
the devices, so the only limit is the total available space.
The required tools are in the lvm2 package and its dependencies. When they're installed, setting
up LVM takes three steps, matching the three levels of concepts.
First, we prepare the physical volumes using pvcreate:
# pvdisplay
# pvcreate /dev/sdb2
Physical volume "/dev/sdb2" successfully created
310 The Debian Administrator's Handbook