Painless Thumbdrive Backups

Raise your hand if you've ever lost (or worried you'd lost) a USB thumbdrive. You spent hours fruitlessly searching the house, and then as you opened the washing machine door, it suddenly dawned on you that perhaps you didn't check your pockets thoroughly when you put this load in.

Fortunately, you have a backup of all the data, right? You religiously mount the drive and copy the data to a backup directory on a regular schedule, no?

That sounds an awful lot like drudgery to me too, and I got into computers to avoid boring work. Naturally, it's a lot more fun to spend some time working out the perfect method for painless thumbdrive backups.

What do I mean by painless? How about a system where you can walk up to your Linux box, plug in the drive, wait for a “backup complete” sound, unplug and walk away? Perhaps a system that keeps its backups orderly (say, the last seven copies)? Oh, and it should handle encrypted thumbdrives as well. And, if you need to recover, it should do both whole-volume replacement and per-file restores.

Not a problem. The key to this system is using udev rules and a simple shell script. The tools already are on your system. In this example, I use a CentOS 4.3 system, though any Linux distribution with a 2.6 kernel should work.

udev is the modern device manager for Linux, replacing the 2.4 kernel's devfs. udev handles all device mapping, including hot plugging of devices. One of its coolest features is it lets you write your own event rules. This article shows you how to craft a rule that automatically fires when you plug your USB thumbdrive in to the system.

These rules are stored in /etc/udev/rules.d (if you're using a different Linux distribution, check /etc/udev/udev.conf for the udev_rules= line, which should point to the rules directory). You can place whatever udev rules you want as text files in this directory, and udev picks them up immediately for use without requiring a reboot.

To write a udev event rule, you first need a unique way to identify the USB device. Most thumbdrives have serial numbers, though not all. Fortunately, even with thumbdrives that do not have a serial number, you can craft udev rules for them.

I use two thumbdrives as examples: a JetFlash JF110, encrypted with TrueCrypt, and a Corsair Flash Voyager. The JetFlash has a serial number; the Corsair does not.

Plug your thumbdrive in, and cat /proc/scsi/usb-storage/*. You should find an entry for it similar to this:

   Host scsi5: usb-storage
       Vendor: Unknown
      Product: USB Mass Storage Device
Serial Number: 85a5b1f2c96492
     Protocol: Transparent SCSI
    Transport: Bulk
       Quirks:

If you have a serial number, skip forward to the “Writing the Rule” section of this article. If you see “None” for the Serial Number, you still can identify the device by using udevinfo. Follow these steps:

1) Look at dmesg's output. Typical output is as follows:

usb-storage: waiting for device to settle before scanning
  Vendor: Corsair   Model: Flash Voyager     Rev: 1.00
  Type:   Direct-Access
  ANSI SCSI SCSI device sde: 2031616 512-byte hdwr sectors (1040 MB)
[...]
sde: assuming drive cache: write through
 sde: sde1
Attached scsi removable disk sde at scsi12, channel 0, id 0, lun 0
Attached scsi generic sg4 at scsi12, channel 0, id 0, lun 0,  type 0

This tells you that /dev/sde is the device assigned.

2) Now, run:

udevinfo -a -p $(udevinfo -q path -n /dev/sde)

and examine the output. Look for these lines:

BUS=="scsi"
SYSFS{model}=="Flash Voyager  "
SYSFS{vendor}=="Corsair "

Now, with either the serial number or the vendor/model combo, you can write the rule. The rule creates a symlink for the device in the /dev tree, for example, /dev/corsair_drive, and then calls the script /usr/local/bin/backup-thumb.sh, which I'll get to in a moment.

Become root (su -), and create a text file in /etc/udev/rules.d called 95.backup.rules. You can use a number other than 95, but keep in mind that udev processes rules in alphanumeric order, and it's better to have local rules processed last.

If you have a serial number, type a rule like this (all on one line) into the file, and save it:

BUS="usb", SYSFS{serial}="85a5b1f2c96492", SYMLINK="jet_drive",
RUN+="/usr/local/bin/backup-thumb.sh jet_drive "

If you're using vendor/model identification, your rule would look like this:

BUS="scsi", SYSFS{vendor}=="Corsair ", SYSFS{model}=="Flash Voyager  ",
SYMLINK="corsair_drive", RUN+="/usr/local/bin/backup-thumb.sh
corsair_drive"

Note that you can string as many SYSFS{} entries together as you need to identify the drive uniquely. Your rule now fires every time you plug in your thumbdrive.

Note: if you have other rules for a device, udev executes the rules in sequence from top to bottom.