[ale] Fwd: Re: [ale] Recommendations on mirrored LVM..?

[Danny Cox]

All,

On Sun, 2002-06-16 at 14:04, James P. Kinney III wrote:
> Yes, a mirror can be added later without blanking the hard drive with
> the original data. 
> > 
> > > Now for the $40,000 question. Can all this be done on a live production
> > > system with data being written to and read from mirrored drives? 
> 
> The LVM has been around since 2.3.30-something in the kernel. It was
> around as a 3rd party patch from 2.1.something. I would put it as nearly
> as mature as some of the HP-UX and AIX versions. The stability of any
> LVM system is directly related to the proficiency of the admin :)

	In the configuration we ship, we only use LVM for snapshots, e.g., to
make backups consistent.  We set up the SW RAID, then the LVM, then the
filesystem above that.

	However, there is a tool (that I've hacked on, beware ;-) that
manipulates RAID groups.  It can change a 3 disk RAID5 to a 4 disk
RAID5.  It can basically map any RAID to any other RAID, import a single
volume into a RAID, or export a RAID to a single volume (if it'll fit). 
It does all this offline, though.  I've heard rumors that it
(raidreconf) is now part of the raidtools package.

	We have envisioned the following scenario in the past:

	1) umount the FS
	2) use raidreconf to add a disk (say a 3 disk RAID5 to a 4 disk RAID5)
	3) tell LVM that it's now larger (sorry, I forget the command for this)
	4) mount the FS (XFS), and use xfs_growfs to increase the size

	Unfortunatly, marketing says they want all this to happen "online". 
Sigh.  Another layer, anyone? ;-)

	As far as stability of the upgrade, LVM should be fine, I believe all
it does is manipulate pointers, and device files.  It should be really
quick during the growth phase, and therefore, very hard to trip up.  I
HAVE see it trip up, however :-(.

	Raidreconf, on the other hand, works fine as long as it's not
interrupted.  It doesn't keep state, and so if interuppted or a power
loss occurs, you're screwed.  Currently, there's:

	a) no indicator of progress (and therefore)
	b) no way to tell it "restart <HERE>"

-- 
kernel, n.: A part of an operating system that preserves the
medieval traditions of sorcery and black art.

Danny


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