Computer data storage - LVM notes

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Computer data storage
These are primarily notes
It won't be complete in any sense.
It exists to contain fragments of useful information.


What is LVM, and what can it do?

tl;dr: you can more easily replace and resize things later, can combine disks, and use copy-on-write snapshots


LVM is a logical layer between physical disks and mountable volumes.

They can be useful for

  • move data to other underlying disks without the filesystem level being aware of this
e.g. preventive replacement of disks
also nice for admins when allocating space to VMs
  • resizing logical volumes, e.g. useful on servers where size requirements are likely to change (only makes sense when your filesystem supports resizes too, but most 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)
    • 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.

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