R Pi Image files - John's Vademecum

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====== R Pi Image files ======

===== Various tips and tricks for working with Raspberry Pi Disk Images =====

==== Taking an image ====

  - remove SD card from Pi
  - insert into PC
  - unmount the image partitions if they get auto-mounted
  - take the image using ''dd''<code bash>sudo dd if=/dev/mmblahblah of=name_of_image.img bs=4M</code>
  - wait..... and wait....
  - this can now be either
      - saved as it is (possibly a big file)
      - compressed with gzip <code bash>gzip name_of_image.img</code>
  - an alternative is to make sure the image is as small as it needs to be, appropriate to the space actually needed by the installed OS.
 
==== Using ''losetup'' to work with the image file ====

The image can be used as a //loop// device and then mounted as a normal disk to allow access to the files etc. 

To access the image as a //loop// device you let ''losetup'' find the partitions automatically and convert them to to loop partitions ''loop0p1'' (the ''/boot'' partition) and ''loop0p2'' the ''root'' partition. You can then mount whichever one you need. 

<code bash>
gm4slv@laptop:~/piimg $ sudo losetup -Pf igate.img
gm4slv@laptop:~/piimg $ lsblk
NAME      MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
loop0       7:0    0   4.3G  0 loop
├─loop0p1 259:0    0   256M  0 part
└─loop0p2 259:1    0     4G  0 part
sda         8:0    0 298.1G  0 disk
├─sda1      8:1    0 235.1G  0 part /
├─sda2      8:2    0     1K  0 part
├─sda3      8:3    0  50.9G  0 part
└─sda5      8:5    0  12.1G  0 part [SWAP]
sr0        11:0    1  1024M  0 rom
gm4slv@laptop:~/piimg $ sudo mount /dev/loop0p2 /mnt
gm4slv@laptop:~/piimg $ cd /mnt
gm4slv@laptop:/mnt $ ls
bin  boot  dev  etc  home  lib  lost+found  media  mnt  opt  proc  root  run  sbin  share  srv  sys  tmp  usr  var
gm4slv@laptop:/mnt $ cd etc/
gm4slv@laptop:/mnt/etc $ cat hostname
igate
gm4slv@laptop:/mnt/etc $

</code>

This allows you to change files (one example is to set a fixed IP address in ''/etc/dhcpcd.conf'') as required.

==== Shrinking the image ====

If the OS had been expanded after install to //fill the whole SD Card space// there is now a lot of pointless wasted space in the image that 
  - takes up disk space to store
  - makes copying to a new SD card slow
  - makes it impossible to copy to a smaller card

The image can be shrunk to a more appropriate size.

The gist of it is [[https://superuser.com/questions/610819/how-to-resize-img-file-created-with-dd|here]]

  * unmount the loop device: <code bash>sudo umount /mnt</code>
  * use ''gparted'' (need to be at a machine with a graphical session) to shrink the partition to a more suitable size, leaving some room for safety
  * dispose of the ''loop'' device <code bash>sudo losetup -d loop0</code>
  * The *.img file is still the same size, but the partitions within the file are smaller, the rest of the space needs to be stripped from the *.img file with ''truncate''
  * Use ''fdisk'' to inspect the image file to see where the partitions start and end 
  * <code bash>gm4slv@laptop:~/piimg $ sudo fdisk -l igate.img
Disk igate.img: 4.25 GiB, 4563402752 bytes, 8912896 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x38ea48c4

Device     Boot  Start     End Sectors  Size Id Type
igate.img1        8192  532479  524288  256M  c W95 FAT32 (LBA)
igate.img2      532480 8912895 8380416    4G 83 Linux
</code>
  * The second partition ends at block **8912895** so you shrink the filesystem to this point (plus one). This requires the calculation of the actual number of bytes, based on the **block size of 512 bytes**. 
  * Either calculate it yourself, or put the calculation in the command:
    * <code bash> sudo truncate --size=$[(8912895+1)*512] name_of_image.img</code>

Voila, a smaller image on file which uses //just enough// space for the //already known to be working// OS




Page Updated: ~~LASTMOD~~

{{tag>linux pi}}
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