[CentOS] LVM hatred, was Re: /boot on a separate partition?

Fri Jun 26 16:51:45 UTC 2015
Gordon Messmer <gordon.messmer at gmail.com>

On 06/26/2015 07:58 AM, Mark Milhollan wrote:
> On Wed, 24 Jun 2015, Gordon Messmer wrote:
>> 1) If you have a system with a single disk, you have to reboot to add
>> partitions for new guests.  Linux won't refresh the partition table on the disk
>> it boots from.
> I'm not sure this is still true, but I use LVM almost everywhere so I
> seldom need to try.

It's definitely still true on CentOS 7.

>> 3) If you want redundancy, partitions on top of RAID is more complex than LVM
>> on top of RAID.  As far as I know, partitions on top of RAID are subject to the
>> same limitation as in #1.
> They look the same to me, and share the same limitations (WRT the PV).

Create a RAID1 volume on two drives.  Partition that volume.

Where is your partition table?  Is it in a spot where your BIOS/UEFI or 
another OS will see it?  Will that non-Linux system try to open or 
modify the partitions inside your RAID?  It depends on what metadata 
version you use.  If you set this up in Anaconda, it's going to be 
version 0.90, and your partition table will be in a spot where a 
non-Linux system will read it.

There's no ambiguity with LVM.  That's what I mean when I say that it's 
less complicated.

The format of MBR and GPT partition tables are imposed by the design of 
BIOS and UEFI.  There is no good reason to use them for any purpose 
other than identifying the location of a filesytem that BIOS or UEFI 
must be able to read.

The limitation I was referring to was that as far as I know, if Linux 
has mounted filesystems from a partitioned RAID set, you can't modify 
partitions without rebooting.  That limitation doesn't affect LVM.

> Either can be partitioned but making more LVs is indeed simpler than
> using DM to partition a partition or MD.  I'd like to use LVM RAID and
> never again have RAIDed partitions, so that I can choose the RAID level
> per LV, alas LVM RAID MDs don't appear in /proc/mdstat so monitoring
> them is somewhat more annoying.
>> 4) As far as I know, Anaconda can't set up a logical volume that's a redundant
>> type, so LVM on top of RAID is the only practical way to support redundant
>> storage of your host filesystems.
> Anaconda has many deficiencies and indeed I am annoyed enough with it
> that I often skip trying to use its new disk manager, but making the PV
> on an MD RAID isn't impossible

I know, that's what I said was the only practical way to support 
redundant storage (when using LVM).

> , or alternatively making the LVs
> redundant after install is a single command (each) and you can choose
> whether it should be mere mirroring or some MD manged RAID level (modulo
> the LVM RAID MD monitoring issue).

I hadn't realized that.  That's an interesting alternative to MD RAID, 
particularly for users who want LVs with different RAID levels.