To expand on this with an example. Adding a new device we'll call sdz to an existing Logical Volume Manager (LVM) Volume Group (VG) called "NAS" such that all the space on sdz is instantly available for adding to any Logical Volume (LV):

  pvcreate /dev/sdz
  vgextend NAS /dev/sdz

Now we want to add additional space to an existing LV "backup":

  lvextend --size +128G --resizefs NAS/backup

*note: --resizefs only works for file-systems supported by 'fsadmn' - its man-page says:

"fsadm utility checks or resizes the filesystem on a device (can be also dm-crypt encrypted device). It tries to use the same API for ext2, ext3, ext4, ReiserFS and XFS filesystem."

If using BTRFS inside the LV, and the LV "backup" is mounted at /srv/backup, tell it to use the additional space using:

  btrfs filesystem resize max /srv/backup

How are redundancy and drive failure handled? The only capacity mix-and-match scheme I have familiarity with is btrfs.

Synology SHR is btrfs (or ext4) on top of LVM and MD. MD is used for redundancy. LVM is used to aggregate multiple MD arrays into a volume group and to allow creating one or move volumes from that volume group.

Comment I responded to was using LVM on its own and I was wondering about durability. The docs seem to suggest LVM supports various software raid configurations but I'm not clear how that interacts with mixing and matching physical volumes of different sizes.