link: https://adamscheller.com/systems-administration/xenserver-local-iso-storage-new-partition/
for posterity…
figure out the name of the volume group (something like name-uuid)
pvscan
create the new volume
lvcreate -L 150G -n ISOs name-uuid
find the volume you just created
lvscan |grep ISO
create the filesystem
mkfs.ext2 /dev/other-name-uuid/ISOs
make the mount point
mkdir /mnt/isos
create the repository
xe sr-create name-label=ISOs type=iso device-config:legacy_mode=true device-config:location=/mnt/isos content-type=iso
mount the disk
mount -t ext2 /dev/name-uuid/ISOs /mnt/isos