Re: BTRFS and debian

[David Christensen <dpchrist@holgerdanske.com>]

On 07/14/18 02:04, Pascal Hambourg wrote:
> Le 14/07/2018 à 02:49, David Christensen a écrit :
> > # file -s /dev/sda1
> > /dev/sda1: BTRFS Filesystem label "po_boot", sectorsize 4096, nodesize 
> > 16384, leafsize 16384, UUID=6ff0dd1d-8d46-454b-bb35-a09afc47145a, 
> > 65490944/999292928 bytes used, 1 devices
> >
> > 2018-07-13 17:39:51 root@po ~
> > # file -s /dev/sda2
> > /dev/sda2: data
> >
> > 2018-07-13 17:39:59 root@po ~
> > # file -s /dev/sda3
> > /dev/sda3: LUKS encrypted file, ver 1 [aes, xts-plain64, sha256] UUID: 
> > 0152d2e2-4cfb-42c4-a121-6fb832962e47
> >
> > The output for /dev/sda2 is not very informative (e.g. clues that it 
> > has dm-crypt, random key, and swap).
>
> As expected. /dev/sda2 contains raw encrypted data which appear as 
> random data. It is the purpose of encryption that one cannot see the 
> real contents.

Okay.


> > > Beware that unlike a UUID or LABEL, a PARTUUID or PARTLABEL is stored 
> > > in the partition table, not in the partition data.
> > > Also note that a DOS partition table entry does not contain a UUID 
> > > nor LABEL, and the PARTUUID is artificially built by combining the 
> > > 32-bit "disk identifier" field in the MBR and the partition number. 
> > > So if the partition number or the disk identifier changes, the 
> > > PARTUUID changes.
> > >
> > > In short :
> > > - if you move the disk contents (including the partition table) to 
> > > another disk, the PARTUUID is preserved ;
> > > - if you move the partition contents to another partition, the 
> > > PARTUUID is not preserved.
> >
> > Thanks for the warning.  My typical use-case is to move the entire 
> > system drive image between various 16+ GB devices, so PARTUUID should 
> > work.
>
> I forgot to mention another case, although you are not concerned :
>
> - logical partitions numbers and synthetic PARTUUIDs may change when 
> creating or deleting another logical partition on the same disk. 

Fortunately, I only need 3 partitions on my system drives.  I have 
always used primary partitions.


> Yet 
> another reason to not use logical partitions and prefer GPT if you need 
> more than 4 partitions.

GPT is nice; I use it when I want a 2+ TB partition (such as my backup/ 
archive/ image drives).  But I have several older computers, so I prefer 
solutions that work with BIOS/MBR.

David