2 - Raspberry PI Community Projects

Allocation
inherit
Read ahead sectors auto
- currently set to 256
Block device 253:0
# lvcreate -n lv_base -L 1G vg_critical
Logical volume "lv_base" created
# lvcreate -n lv_backups -L 12G vg_normal
Logical volume "lv_backups" created
# lvdisplay -C
LV VG Attr LSize Origin Snap% Move Log Copy% Convert
lv_base vg_critical -wi-a- 1.00G
lv_files vg_critical -wi-a- 5.00G
lv_backups vg_normal -wi-a- 12.00G
Two parameters are required when creating logical volumes; they must be passed to the lvcre
ate as options. The name of the LV to be created is specified with the -n option, and its size is
generally given using the -L option. We also need to tell the command what VG to operate on,
of course, hence the last parameter on the command line.
GOING FURTHER
lvcreate options
The lvcreate command has several options to allow tweaking how the LV is
created.
Let's first describe the -l option, with which the LV's size can be given as a
number of blocks (as opposed to the “human” units we used above). These
blocks (called PEs, physical extents, in LVM terms) are contiguous units of
storage space in PVs, and they can't be split across LVs. When one wants to
define storage space for an LV with some precision, for instance to use the full
available space, the -l option will probably be preferred over -L.
It's also possible to hint at the physical location of an LV, so that its extents
are stored on a particular PV (while staying within the ones assigned to the
VG, of course). Since we know that sdb is faster than sdf, we may want to
store the lv_base there if we want to give an advantage to the database server
compared to the file server. The command line becomes: lvcreate -n lv_-
base -L 1G vg_critical /dev/sdb2. Note that this command can fail if
the PV doesn't have enough free extents. In our example, we would probably
have to create lv_base before lv_files to avoid this situation – or free up
some space on sdb2 with the pvmove command.
Logical volumes, once created, end up as block device files in /dev/mapper/:
# ls -l /dev/mapper
total 0
crw-rw---- 1 root root 10, 59 5 oct. 17:40 control
lrwxrwxrwx 1 root root 7 5 oct. 18:14 vg_critical-lv_base -> ../dm-1
lrwxrwxrwx 1 root root 7 5 oct. 18:14 vg_critical-lv_files -> ../dm-0
lrwxrwxrwx 1 root root 7 5 oct. 18:14 vg_normal-lv_backups -> ../dm-2
Chapter 12 — Advanced Administration
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