LVM Extent Calculator
See what a volume group of disks really holds: the extents each disk gives up after its metadata area, the space a logical volume rounds up to, and what stays free.
A physical volume is carved into physical extents of one fixed size, so each disk contributes floor((size − metadata) ÷ extent) whole extents and everything after the last one — the metadata area plus up to one extent of tail slack — is unusable. A volume group is those extents added together, and a logical volume is handed a whole number of them, so its size rounds up to the next extent boundary and the difference is lost. Four 1000 GiB disks with 1 MiB of metadata and 4 MiB extents give 255,999 extents each and 1,023,996 in the group (3,999.984 GiB, one extent per disk short of the raw 4000 GiB); a 3000 GiB logical volume takes 768,000 extents and leaves 255,996 free. This is arithmetic, not an instruction: the page never touches a disk, and thin pools, snapshots, mirrors, stripes, PV labels and the extra metadata volume LVM keeps are not modelled beyond the single metadata field. Sizes are GiB and MiB (powers of 1024), as LVM reports them.