Monday, October 3, 2016

Basics of LVM legends

LVM (logical volume manager) legends : PV is Physical Volume.   Any single disk / LUN on system is identified as PV. It can be raw or f... thumbnail 1 summary
LVM (logical volume manager) legends :

PV is Physical Volume. 
Any single disk / LUN on system is identified as PV. It can be raw or formatted with file system. Raw PV is referred as /dev/rdsk/c0t0d1 (legacy) or /dev/rdisk/disk1 (agile) whereas formatted one is referred as  /dev/dsk/c0t0d1 (legacy) or /dev/disk/disk1 (agile). Check PV name in below output as formatted device.

# vgdisplay -v vg00

--- Volume groups ---
VG Name                     /dev/vg00
VG Write Access             read/write
VG Status                   available
Max LV                      255
Cur LV                      13
Open LV                     13
Max PV                      16
Cur PV                      1
Act PV                      1
Max PE per PV               4355
VGDA                        2
PE Size (Mbytes)            32
Total PE                    4345
Alloc PE                    4303
Free PE                     42
Total PVG                   0
Total Spare PVs             0
Total Spare PVs in use      0

   --- Logical volumes ---
   LV Name                     /dev/vg00/lvol1
   LV Status                   available/syncd
   LV Size (Mbytes)            1024
   Current LE                  32
   Allocated PE                32
   Used PV                     1

   --- Physical volumes ---
   PV Name                     /dev/dsk/c3t0d0s2
   PV Status                   available
   Total PE                    4345
   Free PE                     42
   Autoswitch                  On
   Proactive Polling           On

PE is Physical Extent. 
Its smallest chunk of PV which can be used as block under file system. PV is consist of number of PEs. We always use PV names while using LVM commands. In above example PE size is set to 32MB & total 4345 PEs are available on disk.

VG is Volume Group. 
One or more PV come together to form a Volume Group. This grouping enables to slice down combined
storage capacity of disks to our choice of small volumes. In above example vg00 is volume group made up of single PV & its sliced down to 8 LV (only one shown in above exmaple)

LV is Logical Volume. 
Its a slice of volume group using some capacity of PV to form a smaller volume. Its basically used as a mount point /swap like drives (C:, D:) in Windows. We can see one LV in above example and its details.

LE is Logical Extent.
Same as PE, LE are smallest chunk of LV.

Below tables gives you idea about some numbers related to them:

LVs per VG  range : 1-255, default : 255
PVs per VG  range : 1-255, default : 16
PEs per VG   range : 1-66535 default : 1016

with above table, as max PE size is 64MB and 66,535 PEs max per VG, one can create max of 64x66353=4TB of file system.

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