Computer data storage - LVM notes

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📃 These are primarily notes, intended to be a collection of useful fragments, that will probably never be complete in any sense.

What is LVM, and what can it do?

✎ This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

LVM is a logical layer between physical disks and mountable volumes, that is typically managed and supported within the OS itself, or close enough that the result is still visible as whatever a relatively low-level drive (e.g. block device in linux).

LVMs can be useful for

• splitting up disks for different users (like partitioning, but with some of the below features involved)

• move data to other underlying disks without the filesystem level being aware of this

e.g. preventive replacement of disks, or admins spreading VM's storage around

• resizing logical volumes, e.g. useful on servers where size requirements are likely to change over the long term (only makes sense when your filesystem supports resizes too, but various do)

• combine disks into a single storage space (note: basically like concatentating, so not failure-safe at all)

• mirroring volumes (similar to RAID1)

• making snapshots (online, read-only)
  • often block-based copy-on-write logic - meaning blocks are shared between snapshot and current data until you change the current data.
  • Snapshots are allocated as you create them, and act as mountable filesystems
  • note that in itself offers no protection against drive failure
  • ...and once you run out of copy-on-write space, the snapshots die -- an important detail in your planning
  • useful as protection against (recent) user mistakes, not backup against failure

Linux LVM requires kernel support - that most kernels have.

Linux can boot from LVM columes, and many linux installers understand it.

In windows, there's

• "Dynamic disks" for a long while
• "Storage spaces" since around win10

OSX has Core Storage since Lion

Linux LVM

Terminology

There are three or four major concepts:

• PV: Physical Volume is a physical thing you can hold.

• VG: Volume Group is one or more PVs, which by itself is little more than a named pool of some PVs

• pp: A physical partition is a single partition on a disk (PV).

Yes, you could use a single partition per PV, but in a number of situations it makes sense to subdivide them

• LV: A Logical Volume is a mountable result

made of some combination of underlying linux device names (PV or pp device (verify)) that is configured in a specific way

Exposed LV device names look something like /dev/vgname/lvname

See also:

http://www.debian-administration.org/articles/410