Re: [gentoo-user] LVM and moving drive to another sata port.

[Rich Freeman]

On Fri, Dec 14, 2018 at 9:13 AM Dale <rdalek1967@×××××.com> wrote:
>
> I'm planning to change some connections while swapping and wanted to be
> sure of something before I do any moves like this.  Let's say I move sdc
> and it becomes sdb.  Will LVM still see it the same way?

Yes.  At least by default LVM is going to scan all your drives looking
for LVM PVs and will identify them regardless of what device they are
on, as long as the device gets scanned.

> I suspect it
> tracks the drive by the UUID which stays the same no matter what port or
> sd letter it gets BUT I want to be sure.

It uses a UUID stored in the PV metadata.  So, as long as you don't
confuse it by going and making copies of drives (which duplicates the
ID) without using the LVM tools you'll be fine.

> Am I correct that changing what drive
> connects to what sata port won't matter to LVM and how it sees them?

Yes

> Also, what if I connect one to the PCIe card I have?  Will it still see
> it the same way?

Yes, in general.  The only time you might have an issue is if you use
something more exotic that creates a block device that might not get
scanned by default, but I believe that is just a configuration fix.
So, if you're using iSCSI or something maybe you might need to do a
little work.

>
> Also, I found a wonderful guide for my upcoming move.  It is located here:
>
> http://tldp.org/HOWTO/LVM-HOWTO/removeadisk.html
>
> Scroll down a bit to:  13.5.2. Distributing Old Extents to a New
> Replacement Disk
>
> That covers exactly what I am doing.  Even tho Grant and others say it
> is that easy, I still find it hard to believe.  O_0  I sure am glad I
> was talked into using LVM.  I think it was Alan that first mentioned it
> but not sure.

You wouldn't do this if you're just moving physical disks from one
physical interface to another.

However, if you wanted to migrate data off of one disk and onto
another, this is exactly what you would do, and this is exactly why
everybody always advises people to use LVM (or something like
zfs/btrfs with similar capabilities).  It makes moving data around
almost trivial.  You can migrate your data while your system is in-use
and it isn't a problem at all.

> P. S.  I'm still copying over my /home to the new 8TB backup drive.
> While it is copying at speeds of 20MBs/sec for some files to as high as
> 160MBs/sec for other files, it takes a long time with that much data.
> It is running at a much better speed than it was when I started the
> other thread.

LVM would migrate data more quickly than a filesystem copy, because it
is doing it at the block level.  So, it doesn't matter whether a block
contains 1000 small files or part of one huge file, or filesystem
metadata.  The only thing that should slow down LVM moves would be
disk activity, and I believe you can tune its priority (do you want to
slow down disk access, or LVM copying?).

With a filesystem copy small files will kill your performance in most
cases, with some filesystems being better than others.

-- 
Rich