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Migrating From CentOS-6 To CentOS-7

I asked this in Vol 154, Issue 29:

I've a system that has been running CentOS 6 64-bit for almost
4-years and I want to start the move to CentOS 7 64-bit.

I run software RAID-1 with LVM everywhere except /boot which is a
standard partition RAID-1.

There are 2 areas that concern me greatly about this migration:

1. How do I duplicate my current grub configuration file that
supports a failure of /dev/sda to grub2?

2. How do I install CentOS 7 but use my existing hard drive layout
so I only format specific OS file systems. My drive layout goes back
to the original proposed FHS layout. The separate partitions related
to the operating system are:

/boot        1GB  (standard boot as defined in fstab is /dev/md0)

The following file systems are all defined in volume group
vg_jpdopsys (size is 64 GB)

/ (root)      6GB

/home       4GB

/opt           4GB

/tmp          2GB

/usr            12GB

/usr/local  2GB

/var            8GB

The following file system is defined in volume group vg_jpdswap
(size is 24 GB)

swap          16GB

Plus many other file system dedicated to specific applications and
data all using logical volume management.

When I moved from CentOS 5 to CentOS 6 all of the OS partitions were
formatted except /home.  All went extremely well then.

But I don't see a way to do that with CentOS 7 retaining the
existing disk configuration.

Is there some detailed installation instructions showing some
advanced methods?

TIA

Gene

If you saw the original post you will see that I did clean it up a bit. 
I'm starting this move now understanding that I have until 2020 to do it.

Any suggestions?

TIA

Gene