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Re: LVM/btrfs - Was: Re: firefox-37, where to put


On Fri, Apr 03, 2015 at 03:40:58PM +0200, Petter Adsen wrote:
> On Fri, 3 Apr 2015 09:15:42 -0400
> Gene Heskett <gheskett@wdtv.com> wrote:
> > On Friday 03 April 2015 05:35:49 Reco wrote:
> > > But doing it correct way would probably require using LVM
> > > (snapshots), and LVM is one of those things that are either used
> > > from the start, or not used at all.
> > 
> > Tried twice back when it was all the rage when fedora was fairly
> > new. Both installs self-destructed in under 2 weeks. I've no clue if
> > its ready for "prime time" today or not.  But haveing been twice
> > burnt, I am wary and avoid it.  And of course I eventually got tired
> > of being used for a guinea pig as its survival rates were relatively
> > poor.
> This ties in nicely with something I'm sitting here and wondering
> about right now. I'm preparing to upgrade my home server to Jessie
> today, and at the same time I want to set up something like LVM on the
> system disk (2x250G in mdadm RAID1). The main reason is that I want
> snapshots, and a secondary, nice-to-have-but-not-essential reason is
> future resizing of volumes.

Both are possible with LVM. Snapshots require preliminary space
reservation for keeping the difference between a main filesystem and a
snapshot, and I/O may suffer somewhat, but that's it.
Resizing just works, as long as you don't forget the correct order for
changing the filesystem and the volume. I.e.

1) Enlarge - volume first, filesystem last.
2) Reduce - filesystem first, volume last.

> The alternative to LVM would be btrfs, which would give me RAID1 and
> snapshots, plus subvolumes. I am familiar with mdadm, but I am *not*
> familiar with LVM or btrfs in any way.

I'd stay clear of brtfs if I were you until jessie+1 (I forget whatever
its called) enters freeze. Then you install backported kernel and
*maybe* btrfs would be so kind and would not eat your data.

A simple experiment. Dowload yourself btrfs-tools package from testing.
Unpack it. Locate sbin/fsck.btrfs. Read it, as it's a simple shell
Read a manpage for btrfs-check(8) next.

Observe the following gem:

   Since btrfs is under heavy development especially the btrfs
check command, it is highly recommended to read the
   following btrfs wiki before executing btrfs check with
--repair option:

> What would the experts here recommend? I've been searching for a while
> now, but I haven't found anything recent that applies to both LVM and
> btrfs. I know btrfs is a moving target, is it stable enough to use for
> both it's RAID functionality and the rest? Or would I be better off
> with mdadm and LVM? Which is better to work with?

You have mdadm. Add LVM on top of it. Make sure you have an non-LVM EFI
partition in case of using UEFI (does not apply to BIOS). Don't forget
to add busybox into initrd just in case. Enjoy.

If you really want to try btrfs - stay clear of both mdadm and LVM.
btrfs has own filesystem-aware LVM, and stacking LVMs never was a good


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