Re: [vox-tech] LVM fun

[Samuel Merritt <snmerritt@ucMAPSdavis.edu>]

On Wed, May 15, 2002 at 04:06:37PM -0400, msimons@moria.simons-clan.com wrote:
> On Wed, May 15, 2002 at 01:01:14PM -0700, Samuel Merritt wrote:
> > On Wed, May 15, 2002 at 01:49:13PM -0400, msimons@moria.simons-clan.com wrote:
> > > Is anyone using LVM here?
> > 
> > I use LVM. Kernel 2.4.17, LVM version 1.0.1-rc4(ish)(03/10/2001). 
> 
> Hi Sam,
> 
>   I was digging around in ext2resize man pages, and noticed a lot of 
> references to LVM, I'd also heard something about LVM hitting a 1.0
> status.  So I got curious about it... I haven't looked around for
> information yet...
> 
> - What sources of information on LVM would you recommend?

Pretty much everything I needed to do was in the HOWTO, at 
http://www.sistina.com/lvm_howtos/lvm_howto/ . 

If you use Debian stable, you'll want bunk's packages for kernel 2.4, 
which include up-to-date LVM packages. I've got the following in 
/etc/apt/sources.list: 

deb http://www.fs.tum.de/~bunk/debian potato main 

I recently upgraded the machine to woody, and woody has a slightly newer 
lvm package. 
 
> - Is there much of a performance hit on LVM'd versus non-LVM'd filesystems?

I started using LVM when I got three new drives and put them in a RAID-5, 
so I can't really say. 

If you're interested, my configuration is as follows: 
/dev/hdc1, /dev/hdf1, /dev/hdg1 make up /dev/md0, which is a RAID-5 array. 

/dev/md0 is the only physical volume in volume group "raid" (uber-creative 
name)

Volume group "raid" has two logical volumes on it: 
/dev/raid/usr, 4 GB. 
/dev/raid/home, 130 GB. 

There's about 10 GB free on the RAID-5 for future expansion. I've been 
meaning to move /var onto it so I don't lose my mail spool if /dev/hde 
goes out (has /, /var, and windows (boo, hiss)), but I just haven't gotten 
around to it. 

> - Do you think things are stable enough for everyday use?

The only problem I had with LVM is that you couldn't mount a snapshot of 
an ext3 filesystem. There's a patch for that available at 
http://lists.sistina.com/pipermail/lvm-devel/2001-November/000707.html

I think one piece of the patch has been integrated into 2.4.17, so if one 
hunk fails, that's okay. I looked at the original kernel source; the 
relevant bits are in there.

Hopefully, all the patch will make it into the 2.4 tree someday.

> - Do you think distributions will start installing LVM'd filesystems?

I don't know. It'd certainly be nice if they did. If you need more space
on /home, it's very nice to be able to buy another disk and do the
appropriate magic to add the new disk in. Had 10G on /home, bought a 40G
disk? Now /home is 50G, no messing about with copying a filesystem or
leaving unused partitions around. 

It's a nice abstraction -- you can just consider your total disk space,
instead of having to worry about how much space is on each disk. 

The only problem I see is needing the kernel to be on a regular partition 
in order for LILO or GRUB to work. Otherwise, the bootloader would have to 
scan all the devices, find the LVM info blocks, find the kernel file, and 
load it. And that only works if the BIOS can even see all the block 
devices used - in my case, that would mean LILO would need to know about 
LVM and RAID-5, and be able to still fit in the MBR. 

-- 
Samuel Merritt
PGP key is at http://wwwcsif.cs.ucdavis.edu/~merritt/snmerritt.asc
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