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

[Dale]

Rich Freeman wrote:
> 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.

That's good to know.  I thought it worked that way. 

>> 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.

It's just a plain sata card so nothing fancy.  According to lspci, it
sees the card.  I haven't actually hooked a drive to it yet tho. 


>> 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.

I'm actually replacing a 3TB drive with a 6TB drive.  So, while I'm also
moving drives from one sata port to another, I'll also be replacing a
hard drive as well.  I'm at just over 70%.  It won't be long until it
starts getting to full. 


>> 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.
>

Well, I'm making a backup of /home just in case something goes wrong. 
While I don't plan to change anything, hardware wise, with the drive my
OS is on, I plan to backup /etc and my world file as well.  Just in
case.  ;-) 

Thanks for confirming how LVM works.  At least I am reassured that I can
move things around a bit. 

Dale

:-)  :-)