i am creating snapshot on 3.2TB lvm, but it stops with this

It's telling you you don't have enough free extents to allocate to the new LV. Free some up by shrinking/deleting unused LV, or use the number of free extents you have as an argument to lvcreate (lvcreate -l [#-of-free-extents] -s ....)

It's telling you that you don't have enough free space left to create the 30GB (3TB) filesystem.

Run "vgdisplay VGSANDB" and examine it's output. Specifically look at the information similar to the below:
PE Size 32.00 MB
Total PE 10935
Alloc PE / Size 2592 / 81.00 GB
Free PE / Size 8343 / 260.72 GB

PE Size tells you how large each physical extent is. These are the units used to make your device.

Total PE tells you how many physical extents you have. Multiplying that times your PE Size will tell you how big the overall Volume Group (VG) is.

Alloc PE / Size shows you how many physical extents you're using already and also calculates PE * PE Size to show how much space you're using.

Free PE / Szie shows you hwo many physical extens are NOT in use yet and also caclulates PE * PE Size to show how much unused space is available.

The message you got is telling you the last number (Free PE / Size) is too small to allow you to do what you're trying.

30gb != 3tb

D'oh!

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