[CentOS] RE: Setting up RAID using mdadm on a proliant DL320 G4

Sun Sep 2 16:49:58 UTC 2007
William L. Maltby <CentOS4Bill at triad.rr.com>

On Wed, 2007-08-29 at 16:38 -0400, Ross S. W. Walker wrote:
> > From: centos-bounces at centos.org 
> > [mailto:centos-bounces at centos.org] On Behalf Of 
> > israel.garcia at cimex.com.cu
> > 
> ><snip>

> ... does anybody knows how to setup a 
> > software linux RAID on a proliant DL320 G4? 
> <snip>

> When I setup my OS HD in a RAID1 I followed this recipe:
> <snip>

> 5) create 4GB LV called swap, formatted swap

OK. This particular item has aggravated me over many different posts and
I've withstood the urge to holler "WHY"?

LVM adds another layer of (unnecessary) overhead. For swap to be usable,
it must be formatted first, so it can't be expanded on-the-fly with LVM
in use[1]. Further, expansion on-the-fly can be (effectively)
accomplished by adding another partition or swap file (ugh! more
unnecessary overhead) as needed when LVM is not in use.

Extra swap partitions/files can be easily enabled/disabled as desired
on-the-fly without the extra overhead.

What's more, by having multiple swaps predefined and active at different
priorities at boot-time, unexpected short-term surges in the use of swap
can be gracefully accommodated. Since this requires no more space, has
less overhead, is not dependent on anything other than primitive block
device handling and seems to have as much flexibility and on-the-fly
growth capability as one could want, I always asked "WHY?".

I am all ears (although it may not seem so) hoping to learn something

> <snip>

[1] Although the mkswap man page discusses the formatting in minimal
detail, it *appears* that a "tag" and bit-map of used blocks is
needed/created when mkswap is run. If the vgextend command is used,
after appropriate pvcreate functions, will the extension be properly
formatted and used? Since you would not be adding a new swap
partition/file, but extending an existing one, I suspect that the new
space may go unused until a rerun of mkswap over the extended volume
group is done, during which time the swap is not available at all. I
don't know for sure and am too uninterested to research it that far when
there seems no benefit from the use of LVM in the first place when all
the potential benefits of LVM (only extensibility when in the context of
swap) can be obtained other ways.