JP Vossen on 11 Jan 2009 13:37:42 -0800

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Re: [PLUG] Debian/Ubuntu installer LVM issue

> Date: Sat, 10 Jan 2009 01:27:59 -0500
> From: Matthew Rosewarne <>
> On Friday 09 January 2009, JP Vossen wrote:
>> Huh?!?!?  So it turns out this is a really stupid bug in the Debian
>> installer where it uses totally different bases to calculate the
>> numbers:
>> *+in+volume+group%22
> I always "cheat" and set up my LVM on tty2 with the standard tools instead of 
> using partman-lvm.  After manually making my LV's, I go back to the 
> partitioner and use it for making & mounting the filesystems for them.

I do this so rarely that I forget how to do it at the CLI between 
installs.  (Remember, I'm an old guy. :)

>> You can get the "#Free PE" by going to another PTTY and running
>> lvdisplay, or just trying it, then going to ALT-F4 and reading the error.
> Why not just run `vgdisplay $VG`?  It tells you not only how many free PE you 
> have, but also their size.  You shouldn't really need to use pvdisplay unless 
> you're planning to remove a disk from a VG.

You're right that it has a more friendly view that might be easier to 
use.  I didn't think of that because I can't use vgdisplay on a VG I 
haven't created yet...  But once anything (like lv_swap_1) is there, it 
will show what's there and what's left.  Thanks.

>> This "method" just worked on a test machine, but I'm pretty sure I'm
>> doing something amazingly stupid with that math, and that several folks
>> are about to slap me around for missing something blindingly obvious to
>> a 3rd grader.  Bring it on...  :-)
> Instead of sizing your root LV as:
> 	all available PE
> 	- 2GB
> 	----------------
> 	LV size
> I would actually suggest sizing it (and any other) LV's as
> 	how much you need to just contain the data
> 	+ a fairly conservative estimate of how much more data will accumulate
> 	+ some small arbitrary number of GB.  If in doubt, use a die.
> 	----------------------------------------------------------------------
> 	LV size
> This gives you the flexibility to grow the LV('s) if necessary or allocate the 
> free space to other things, such as snapshots.  It's much less hassle to grow 
> an LV and its filesystem than it is to shrink them.

True.  I do it my way because I am already maxing out the space and 
there is no way to add more.  I have 1u rack-mount pizza-box servers 
that only fit 2 disks, and I'm using both in a mirror.  Then I have a 
small /boot/ (habit left over from old BIOSs that couldn't boot large 
disks) and the rest is /.  So there will never be a need (or 
possibility) to expand...  In other cases, your method is better.

The *only* reason I'm using LVM at all is for the snapshots.  None of 
the other LVM goodness applies, as noted above.

JP Vossen, CISSP            |:::======|
My Account, My Opinions     |=========|
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