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Re: Any advice for a user about to use LVM for the first time?

On 02/01/15 11:57 AM, Kevin O'Gorman wrote:
On Fri, Jan 2, 2015 at 8:50 AM, Gary Dale <garydale@torfree.net <mailto:garydale@torfree.net>> wrote: 

    On 31/12/14 04:57 PM, Kevin O'Gorman wrote:

        I've just gotten 4 4TB drives to replace my 4 2TB drives. I'm
        wanting to have one normal 4TB drive and one logical 12TB
        drive, so I will make three physical drives into one group,
        one logical volume and one partition support the big
        partition.  My system actually resides on a fifth: an SSD
        drive.  I am not interested in RAID, and I'm not sure striping
        would even help.  I just have gigantic files I need to create
        and process once in a while, so it's really temporary space.

        I do want to insulate the one drive from any failures on the
        other three.  That data is not at all temporary, but it is
        backed up regularly.  I want to limit it's failure profile.

        I've read through some documentation, including
        http://www.debian-administration.org/article/410/A_simple_introduction_to_working_with_LVM
        So I think I know how to do it.  I'm just not sure I know how
        to do it _best_.  I'm a bit daunted by the size of
        /etc/lvm/lvm.conf, and wonder if the defaults are going to
        work for me.

        I'm about to start a backup of the existing system. It will
        take a while.  I wonder if anyone has wisdom they'd like to share.

    I've never had any use for LVM. With 4 x 4T drives, why not create
    a single 12T RAID 5 array, or use ZFS or BTRFS as others have
    suggested.
I've decided on mdadm, as you suggest. I understand RAID, just not the details of mdadm. Both ZFS and BTRFS are unknowns to me, so I'll avoid them for now just because I'm lazy. Would either of them span multiple drives, or would I have to have a RAID anyway?
MDADM RAID is simple, although it may not be easy to set up when you already have 4 drives in your system. Most MBs only allow for 6 SATA devices. However you can set up the array with a couple of simple commands. And it can be moved between machines because it doesn't depend on the hardware.
Both ZFS and BTRFS allow just about everything that RAID and LVM offer plus more. BTRFS is the touted as being the next standard Linux file system. If I understand it correctly, you can upgrade ext* to btrfs in place, add new drives into the file system and remove the old ones (using the correct file system commands).
Of course, nothing is 100% safe but is there a point in telling you to backup 8T of data first? MDADM RAID would allow you to copy the data to the new array before removing it from the old disks. Btrfs would allow you to revert if moving data off a drive runs into a problem. However if your current data doesn't have redundancy, a failure during the move could still mean data loss.
This isn't the place for either a RAID or file system tutorial, but there are lots out there on the web. Search engines will find them for you.
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