RAID-10 and Volumes

[J. Epperson]

On Wed, December 31, 2008 11:47, Landreth, Kevin wrote:
>> But perhaps I'm not up to date on the current state of LVM.  The tdlp
>> HOWTO has not been updated in two years, and the arguments in favor
>> there are rather 90-ish themselves.  Are there more recent pages on the
>> benefits you could point me to?
>
> Well, LVM2 hasn't changed too drastically (in general, more features have
> been added though) but there are underlying benefits that may not be
> apparent as most of the "sales material" is based around dynamic
> allocation and resizing on the fly.  Let's take your latter example of
> using partitions as optimal and I will try my best to explain this use
> case (which is more common at its roots, not the specifics).  This only
> cover one of the many many use cases of LVM...
>
> Let's say that you are happy with your in server raid array with LVM, 4
> or 5 LVs (/, /usr, /financial, /home,/var, as an example).  You either
> are running low on space or your boss says that all financial data needs
> to be moved to the SAN.  After a quick edit to lvm.conf (PV filters don't
> search for LUNs by default) , you can now add the LUN from the SAN to
> your Volume group.  It's obvious that /, /usr, /home, and /var don't need
> to be on the SAN, so we only pvmove /financial because it is a non
> interrupting process.  Now we have both local and remote storage and you
> didn't have to schedule a maintence window, didn't have to re-configure
> /etc/fstab, didn't have to re-configure all the workstations to use a new
> file server, etc.  Same applies to adding a MD1000 or MD3000
>
> Now watch this (much more complicated)...you have a bigger server you
> want to migrate to.  You can actually pvmove everything to the SAN if you
> wish, and netboot the new server and everything will be peachy!  You can
> now seamlessly upgrade the server hardware without changing any of the OS
> features!
>

I'd actually forgotten pvmove, have not touched it in many years (AIX
4.2?), but recall using it very successfully with ordinary filesystems
containing ordinary files.  Most of my Linux machines with dynamic space
requirements these days are in Oracle RAC clusters with shared LUNs, and I
still don't think LVM would be advisable, even if workable, for those
situations.

But your points are well taken, and thanks for your collegiality in laying
out the scenarios.