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What are extents?

For example, the command:

vgcreate -s 16M vg1 /dev/sda7 /dev/sda8

creates a Volume Group, vg1. Am I right in saying that extents are like buckets?

i.e. we're setting a 16M bucket size?

And can anyone refer me to some simple documentation on extents. E.g. this page:

https://www.centos.org/docs/5/html/Cluster_Logical_Volume_Manager/VG_create.html just mentions

This extent is the minimum amount by which the logical volume may be increased or decreased in size.

but doesn't say why it's called an extent, whether a Volume Group extent is the same as a Logical Volume extent, etc...

Any help?

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LVM breaks up each physical volume into extents. A logical volume consists of a set of extents. Each extent is either wholly unused, or wholly in used by a particular logical volume: extents cannot be subdivided. Extents are the elementary blocks of LVM allocation.

I don't know why the word extent was chosen.

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