A FOB STORY: some notes on bootable flash drives

Persistence

Unlike consistency (“A foolish consistency is the hobgoblin of little minds…”  Ralph Waldo Emerson ) persistence can be a wonderful thing.

Live bootable media (CDs, flash drives, etc) are ephemeral.  They are either literally read-only, or run as if they were.  Any changes made to the running system – Ubuntu Linux in these examples – disappear at shutdown and aren’t around the next time the flash drive is booted.  That can be a good thing, but there are times when persistence between boots can be useful – when a live distro is updated, for instance, or if a needed wifi driver or useful app  or utility has been added.

Fortunately, a mechanism for this has been added to newer versions of Ubuntu.  There are ways to store persistent data on alternate media – the hard drive on the target system, for instance, or another flash drive – but for our purposes we’ll put the persistent storage on our flash drive.

A somewhat generic, live CD oriented discussion:  Live CD/Persistence

We’ll use the instructions there to create a loopback file with an internal ext3 filesystem.  The exact location of the file is optional, but the name is not.  It must be casper-rw.  Since my flash drives are multiboot, with several Ubuntu distros, I use several persistence files located in a directory named for the distro.

The commands below create a 128MB file with an ext3 filesystem.  It may be necessary to remount the flash drive read/write.  The path to casper-rw will vary depending on the user’s preference on location and where the flash drive is mounted.  That further varies depending on what’s being used to do the creation formatting, an installed Ubuntu, a live CD, a live flash drive – either directly booted or a loop-mounted ISO.  See previous page “Standalone Grub2…” for path examples and remount read/write command.

With the flash drive mounted in a running Ubuntu virtual machine:

$ dd if=/dev/zero of=/media/wdw/307E-B176/bootable/persistent/ubuntu-14.04.1-desktop-amd64/casper-rw bs=1M count=128
128+0 records in
128+0 records out
134217728 bytes transferred in 0.947819 seconds (141606919 bytes/sec

$ mkfs.ext3 /media/wdw/307E-B176/bootable/persistent/ubuntu-14.04.1-desktop-amd64/casper-rw
mke2fs 1.38 (30-Jun-2005)
/media/hda1/casper-rw is not a block special device.
Proceed anyway? (y,n) y
Filesystem label=
OS type: Linux
Block size=1024 (log=0)
Fragment size=1024 (log=0)
32768 inodes, 131072 blocks
6553 blocks (5.00%) reserved for the super user
First data block=1
16 block groups
8192 blocks per group, 8192 fragments per group
2048 inodes per group
Superblock backups stored on blocks:
   8193, 24577, 40961, 57345, 73729

Writing inode tables: done
Creating journal (4096 blocks): done
Writing superblocks and filesystem accounting information: done 

The boot stanza for the above distro + persistence file looks like this.  This is the version booting Ubuntu that’s been copied from the contents of the ISO.  A loop-mounted and booted ISO would use similar persistence entries, but a different boot configuration.  See the previous page “Booting ISOs…”

menuentry "Boot Ubuntu 14.04.1 with persistence" {
    set gfxpayload=keep
    linux /casper/vmlinuz.efi file=/cdrom/preseed/ubuntu.seed boot=casper persistent persistent-path=/bootable/persistent/ubuntu-14.04.1-desktop-amd64 splash --
    initrd /casper/initrd.lz
}

When it comes to persistence files, size matters.  Larger files will slow down booting the particular distro on the flash drive.  The required size will vary depending on need: a few wifi drivers, or a full distro update.

In the interest of speed, it isn’t necessary to use persistence every time a particular distro is booted.  If the target machine doesn’t require updated drivers, just boot the installed distro without peristence.  Multiple grub.cfg boot stanzas can be used for the same distro: one with and one without persistence.

For that matter, persistence isn’t necessary to simply save or add data to the flash drive.  Make the running flash drive writable and just copy data to it.  Since one of these fobs is always on my key chain or in my backpack, I have an “/export” directory where I back up everything and keep frequently used downloads, images, videos…  Where persistence is useful is when the running distro itself has been modified – with added or updated drivers, for instance.

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