could not figure out how much is one extent? - partition

Question

i got the homework from Devops school and one of LVM question below , i am confused with extents , teacher said that it should be calculated like this : 20x25 = 500 mb --> so the partition supposed to be made about 600 mb with extra space ! But from my google researches i found that 1 extent = 4 MB , 25x4 = 100 LVM=80MB ???
Create logical volume "datashare" inside volume group called "datagroup"
Create volume group "datagroup" from partition using 25M extents
Create logical volume with 20 extents /dev/datagroup/datashare

Answer 1

The disk I am using is 20GB in total size.
[root#centos7 ~]# lsblk /dev/sdb
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
sdb 8:16 0 20G 0 disk
I create my first Physical Volume.
[root#centos7 ~]# pvcreate /dev/sdb
Physical volume "/dev/sdb" successfully created.
I create my first Volume Group setting 32MG as the Physical Extentsize (PE) size, default 4MB, this defines how the space is being allocated (chunks of PE size).
[root#centos7 ~]# vgcreate -s 32M myvg /dev/sdb
Volume group "myvg" successfully created
So my /dev/sdb disk size is 20GB, then I created the PV pvcreate, after this I created a new Volume Group with the Physical Extentsize (PE) size of 32MB.
Then I confirm this by the PE Size column.
[root#centos7 ~]# vgdisplay myvg
--- Volume group ---
VG Name myvg
System ID
Format lvm2
Metadata Areas 1
Metadata Sequence No 1
VG Access read/write
VG Status resizable
MAX LV 0
Cur LV 0
Open LV 0
Max PV 0
Cur PV 1
Act PV 1
VG Size <19.97 GiB
PE Size 32.00 MiB
Total PE 639
Alloc PE / Size 0 / 0
Free PE / Size 639 / <19.97 GiB
VG UUID m3wDvh-i0aH-5Zr2-0ya7-1GaA-mLb2-Umd9x3
SO here is the math disk 20GB to MB (20 * 1024 = 20480).
Then 20480 MB (Total disk 20GB) / 32 MB (The size I wanted my PE) = 640 (vgdisplay shows Total PE 639 because rounded values probably we did not have exactly 32 MB to have the last PE to have exactly 640 as expected.)
So if you need to create a new Logical Volume (LV):
You can use PE (which is 32MG and we have 639 available to use), so let say I want a new LV with 50 PE, 50 * 32 MB (PE size) = 1600 MB / 1.6 GB. (Please note Current LE 50 from PE it changes to Logical Extent)
[root#centos7 ~]# lvcreate -l 50 -n mylv1 myvg
Logical volume "mylv1" created.
[root#centos7 ~]# lvdisplay /dev/myvg/mylv1
--- Logical volume ---
LV Path /dev/myvg/mylv1
LV Name mylv1
VG Name myvg
LV UUID BuQsPK-UKWL-tdVv-bFkR-X2md-zG3o-xzQIKk
LV Write Access read/write
LV Creation host, time centos7, 2021-10-10 20:38:33 +0000
LV Status available
# open 0
LV Size 1.56 GiB
Current LE 50
Segments 1
Allocation inherit
Read ahead sectors auto
- currently set to 8192
Block device 253:0
or
You can ask LVM to create an LV with a specific size like 950 MB so 950 / 32 MB (PE) = 29.6875 but it can not use 29.6875 PE because it needs to be rounded up to use 30 PE (Current LE 30).
[root#centos7 ~]# lvcreate -L 950MB -n mylv2 myvg
Rounding up size to full physical extent 960.00 MiB
Logical volume "mylv2" created.
[root#centos7 ~]# lvdisplay /dev/myvg/mylv2
--- Logical volume ---
LV Path /dev/myvg/mylv2
LV Name mylv2
VG Name myvg
LV UUID eJrAY2-Pb1x-VBbq-k8cI-vIlq-Tg3s-CsRFsB
LV Write Access read/write
LV Creation host, time centos7, 2021-10-10 20:40:33 +0000
LV Status available
# open 0
LV Size 960.00 MiB
Current LE 30
Segments 1
Allocation inherit
Read ahead sectors auto
- currently set to 8192
Block device 253:1