This was tested on RHEL7 but should work on any recent Linux.
In case you have a too large disk or you want to roll back an expansion because for instance the need of extra disk space isn't relevant anymore, you can follow this guide.
! Always make sure you have a backup, these are dangerous commands to run that can lead to data corruption
There are a lot of tricks you can do but these ones are tested. Shifting around with new disks to swap should be the safest way but Red Hat's implementation of Grub and initramfs has some drawbacks which during my tests lead to an unbootable machine. This article describes how I did a move (over network if you like) in a Debian-based set up.
First things first, resize the actual file system(s). If you can't unmount the partition, start from a rescue disk (gparted-live for instance) to do this part and return to the installed system afterwards. Take a margin of 10% to make sure we don't have any conversion issues resulting in dataloss (repeat this for all partitions)
If you are on xfs or another filesystem that doesn't allow shrinking you can use fstransform (actually you can always use this). fstransform is in the EPEL directory but you can just download the RPM and install it with yum install too.
If you want to keep xfs
fstransform /dev/[volumegroup]/[logicalvolume] --new-size=[size] xfs
If you rather would have ext4
fstransform /dev/[volumegroup]/[logicalvolume] --new-size=[size] ext4
e2fsck -f /dev/[volumegroup]/[logicalvolume]
resize2fs -p /dev/[volumegroup]/[logicalvolume] [size]
resize2fs -p /dev/mapper/vg_system_lv_tmp 900M
Next we are going to reduce the LVM volume (if you can't manage the downtime, this part you can do with a mounted FS) (repeat this for all partitions)
lvreduce -L [newlvsize] /dev/[volumegroup]/[logicalvolume]
lvreduce -L 900M /dev/mapper/vg_system_lv_tmp
Next if you want to drop a disk, now is the moment to clean it out. (if your LVM is on one disk, you can skip this part)
pvmove /dev/[the disk you want to remove]
vgreduce [volumegroup] /dev/[the disk you want to remove]
pvremove /dev/[the disk you want to remove]
Now you can drop/reuse this disk.
To shrink the LVM part on a disk with partition table you should first check the used and total extensions and ask LVM to move all extensions to the front of the LVM partition
pvmove --alloc anywhere /dev/[partition]:[alloc PE + 1]-[total PE - 1]
Now check if all used extends are on the front of the disk and the only free portion is on the end. If not repeat the previous pvmove command until you get the desired result.
pvs -v --segments
This is how it should look
And now you can resize the volume group to match the wanted disk size
pvresize --setphysicalvolumesize [size] /dev/[lvm partition]
pvresize --setphysicalvolumesize 38G /dev/sda2
Next you can resize the partition table to become small enough to fit the end size of your disk and next you can resize the virtual disk. Check the extends of an other virtual machine with the wanted disk size if you want to push it to the limit, otherwise keep a margin. I used 40G as an example.
*d* -> select partition 2 if that is the LVM one
*n* -> create a new primary partition 2 with the default suggested start sector but for the end sector select 83886079 (for a 40G total disk with 500M boot partition)
*:wq* to save
Now resize the VM disk to the same size (40G in this example). Next you can use lvextend and resize2fs to fill up the margin again to get the desired result.
Since it seems impossible to shrink a disk in hyper-v (Gen 1). I added a 40G disk, started the live cd again and did:
dd if=/dev/sda of=/dev/sdb
Next I just removed the first disk (if you want to be safe, press remove but select "no" when hyper-v asks if you want to remove the disk from the host too) and made the new disk the primary disk and marked it "contains the operating system for the virtual machine" and it booted fine.
pvresize /dev/[lvm partition]
lvresize -L [oldlvsize] /dev/[volumegroup]/[logicalvolume]
And finally delete the old disk from the host if you kept it. For this go to the host running the VM, go into the directory you find when you open the disk settings of the VM and just delete the old disk with shift+delete.