What are Physical Extents (PEs) in LVM?

PEs are the fixed-size blocks (typically 4 MiB) a Volume Group is divided into; every Logical Volume is allocated as a whole number of these extents.

Inside a VG, space isn't handed out byte-by-byte — it's chopped into equal Physical Extents, and LVs are built from a whole number of them. Think of the VG as a sheet of graph paper: each square is one PE, and an LV is a region of squares. This is why LVM can resize so cleanly — growing a volume just means assigning it more squares from the free pool.

Key facts:

• Default PE size is 4 MiB; you can set it at VG-creation time.
• An LV's size is always rounded up to the nearest PE boundary — ask for 300 MB and you get the next whole multiple of 4 MiB.
• The number of free PEs in a VG is your real "how much can I still allocate?" budget.

pvdisplay /dev/vdb1     # PE Size, Total/Free PE for this PV
vgdisplay vg01          # Total/Free/Allocated PE for the whole group

Example vgdisplay excerpt:

PE Size            4.00 MiB
VG Size         1012.00 MiB
Total PE         253
Alloc PE / Size   75 / 300.00 MiB
Free  PE / Size  178 / 712.00 MiB

Here 75 of 253 extents (300 MiB) are allocated, leaving 178 free PEs — about 712 MiB still available to give to an LV. You can allocate an LV by extents directly with lvcreate -l 32 ... instead of a byte size.