[rhelv6-beta-list] My first experiences with RHEL6 beta

[Bryan J. Smith]

Keeping the context of the original discussion in-mind ... ;)

On Tue, 6/15/10, Chris Adams <cmadams at hiwaay.net> wrote:
> IMHO the biggest drawback with LVM and filesystem changes
> right now on Linux is that you cannot shrink a mounted
> filesystem (at least not ext*), only expand.

This is a limitation of e2fsprogs, not LVM.  I do not believe it has
anything to do with LVM.  Feel free to correct me if I am wrong, but
I believe this is very much the case.

In fact, remembering the context of the original discussion, 
how is LVM "drawback" here at all?  I.e., with regards to using
the raw, legacy PC BIOS/DOS Disk Label (aka MBR Partition Table)
is LVM a drawback?

> That makes many live changes such as re-allocating space from one
> use to another impossible.

Moving Physical Extents (PEs) around is very much possible with 
LVM2/DM at any time, _regardless_ of filesystem.  All that is happening
is that kernel facilities are changing the physical blocks of logical
blocks that are linearly addressed via DeviceMapper and other
facilities.

If the filesystem itself has issues with its own meta being changed,
such as in reducing Ext2/3 live (or reducing GFS/2 at all), that's
another story, and has nothing to do with LVM2, DeviceMapper, etc...

> With a swap file, you can create a new file on another filesystem,
> swapon the new file, and swapoff an old file, essentially moving free
> space in one filesystem to another.  You can't really do that with a
> swap LV.

???  I have no idea where this logic came from  ???

I don't even have to "swap off."  I can move where the PEs of the swap
LV, "live."  Of course, if I increase the size of the LV, I have to
do a "swap off" then "mkswap" and then "swap on."  Of course, I could
just allocate some PEs in my VG for a 2nd swap LV, and then "mkswap"
and "swap on" instead.

Absolutely no idea where these comparisons are coming from, and the
logic seems to suggest the opposite of reality.  I just did a presentation
at a local LUG this weekend, and I was surprised how many people were
not familiar with what kernel 2.6 and DeviceMapper were doing underneath.
It was a great interaction, and a great "pre-presentation" to LVM which
I'm doing next month.
 
> Really?  I've used DECs for years and didn't know that.

Yes.  The nomenclature came from Digital, even if aged.

Of course, the original versions of Linux LVM (and even GFS version 1)
come from the mid-to-late '90s.  So the terminology was not changed,
only the capabilities expanded.


-- 
Bryan J  Smith             Professional, Technical Annoyance 
Linked Profile:           http://www.linkedin.com/in/bjsmith 
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