Re: [gentoo-user] backing up a partition

[Bas Zoutendijk]

Dear Philip,

On Fri 24 Aug 2018 at 04:32:23 -0400, Philip Webb wrote:
> I want to make a copy of a partition which I can use to replace it,
> if some catastrophe damages the partition or wipes it out ;
> it needs to be byte-byte identical, incl all permissions.
>
> Can I use 'dd' ? -- eg 'dd if=/mnt/xxx of=/mnt/yyy',
> where the partition has been mounted at  /mnt/xxx
> & a USB stick has been mounted at  /mnt/yyy .  Will that do the job ?

  If you want  to make a byte-by-byte copy  of a partition,  the easiest
way  in   my  opinion  is   to  dd  the   (unmounted)  partition,   e.g.
if=/dev/sda1.  You  could  also do  that  with  the  entire drive,  e.g.
if=/dev/sda.

> There seems also to be an issue re 'bs=<some number of bytes>' :
> what size is best ?  i plan to use USB 3.0 for quicker copying.

  For optimal  performance the block size  should be  a multiple  of the
device’s physical  block size.  Old hard drives  had 512-B blocks,  more
recent  ones  have  larger  4-KiB  physical  blocks  that  are sometimes
software-emulated to  appear as  512-B blocks.  In  my experience  it is
faster to use even larger block sizes; I usually use bs=1M.

> Does it matter how the USB stick is formatted ?
> Can I use a raw stick with the usual default VFAT formatting ?
> Might it be better to replace that with a Linux FS, eg Ext2 ?

  The lowest overhead  is when you write the back-up  to a raw partition
of  the same  size,  e.g.  of=/dev/sdb1.  You can  also  write  it  to a
filesystem,  which  might make  it easier  to manage  multiple back-ups.
Since all permissions etc.  are stored  in the back-up file,  the choice
of file system does not really matter,  though the back-up file might be
too large for VFAT (max ~4 GiB).

                                                              Sincerely,

                                                                 Bas

-- 
Sebastiaan L. Zoutendijk | slzoutendijk@×××××.com