From: Jakma, Paul (Paul.Jakma at domain compaq.com)
Date: Tue 28 Sep 1999 - 16:19:50 IST
>
> What's LVM ?
LVM is way way way.... (breath) way cool.
Linux LVM introduces a layer of abstraction on top of physical devices.. to
create logical devices.
basically you can group real devices into a 'pool'. And from this pool you
can carve out logical devices, and it's these logical devices where you put
your filesystem. ie something like:
| logical | log. | log | log. |
---------------------------------------------
| pool |
---------------------------------------------
|physical | phys. | phys | phys |
---------------------------------------------
the cool bit is you can add/remove physical devices to/from the 'pool'
without affecting the logical devices in that pool. so you can remove a disk
from a pool and your filesystems won't notice at all.. they can stay
mounted. Also by adding disks you can increase the size of the pool, and
also (if you choose) of existing logical devices within the pool.
(caveat: pool must always be big enough to hold logicals within it)
if you have a resizeable filesystem (or resizing tools) then you need never
worry about partitioning again, cause you'll be able to do it all on the
fly.
Another cool thing you can do, (and on topic for this thread), is you can
replace a disk without having to copy files.. you just add the new disk to
the pool, then remove the old disk. hey presto, your files have moved -
without even needing to be unmounted! (in theory.. never mind extents)
Other cool things: you can tell LVM to use raid0 across a couple of disks.
So your pool is automatically RAID'd.
Most commercial Unix's have had this for quite a while, either native or
supplied by Veritas (veritas.com). Eg Solaris, Digital Unix - which has
'Advanced FS', resizable journalled.. etc. HP-UX, AIX, etc..
See http://linux.msede.com/lvm/. My only gripe is that the LVM code only
supports RAID0/1, no RAID5... hopefully LVM will be integrated with
linux-RAID soon. But Heinz wants RAID integrated into LVM, and Ingo wants
LVM integrated into RAID. :(
But once it does linux will be a very advanced storage server platform.
-paul.
(trying now to calm down from excitement of thinking about LVM) :)
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