Recovery

ihacku 2015-03-15

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Not to be confused with NVIDIA's APX mode, a.k.a. "NvFlash recovery mode"; APX mode is lower level and more primitive than Android Recovery.

In Android devices, Recovery is an Android-based, lightweight runtime environment separate from and parallel to the main Android operating system. Recovery's original purpose was:

to apply software updates to the device, e.g. OTA updates, and
to erase user data and cache, e.g. for troubleshooting or preparing the device for resale (factory reset) and
to run (another) external tool from microSD flash memory

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What is Recovery

Recovery is stored in a disk partition separate from the main Android partitions (boot/kernel, root/system). It contains its own Linux kernel, separate from the kernel of the main Android system. Because of separate kernel+recovery_apps, the device is able to boot into the recovery mode even if the main system is broken, allowing the user a set of tools to fix the device. The recovery is completely independent of the Android system.

The bootloader determines whether to boot Android or Recovery.

Custom recovery apps

The open-source community has extended the original Android Recovery system, to add other features:

system backup and restore (NANDroid),
the ability to apply Android changes that aren't approved by the manufacturers or carriers,
improved user interface, including using the touch screen instead of only the volume and power buttons, and
runs adbd, to support ADB connections.

The most popular custom recovery is ClockworkMod Recovery. Others are:

Custom recovery on external microSD

If extended recovery is needed, many manuals on the Net requires user to reflash recovery partition with custom recovery as first step. This is somewhat dangerous and inexperienced user may brick the device. Even built-in recovery is very simple (if there is one at all), it is able to run update.zip from external flash (usually microSD flash card). This allows you to run the custom recovery application from external flash without actually flashing it to the device itself. You have to read documentation for your device to find where to download correct recovery and how to run it.

Entering recovery mode

This is possible to enter recovery mode by pressing some button combination during boot sequence of the device. The key combination differ from phone to phone, but manufactures usually has similar invocations for similar phones. You have to read a page with instruction for your phone to make sure that you are not doing something weird. When these keys are pressed, the device does not perform normal boot sequence. Instead of this, another partition is picked and kernel+recovery_apps is invoked from this recovery partition, so the rest of the flash is untouched (see custom recovery_apps list above). The combination of pressed keys has nothing to do with the recovery kernel+recovery_apps (ie. what is flashed to the recovery partition, this will be run). Sometimes there is no recovery flashed, so even keys are pressed, no recovery mode is invoked. You may still be able to flash own recovery to the device by Odin mode and use it later.

Samsung Galaxy devices

Some Samsung Galaxy phones has own recovery mode with simple ClockworkMod Recovery (CWM) in its stock firmware. Extended CWM could be flashed via Odin mode or from stock recovery mode. You may enter recovery mode this way (see documentation for your device to make sure you are right):

power off the device
press Volume Up + Home + Power ON buttons at the same time and hold them pressed
- most of Glaxy devices boots twice to enter recovery mode, so just hold keys pressed and wait
release buttons when CWM announce itself
use Volume Up/Down and Home button to pick menu entries

Some Samsung devices (notably Samsung Galaxy Tab series) has no Home hardware button. They have only software buttons or none at all (they are emulated by software on the touchscreen). These devices may require only Volume Up + Power ON to be pressed during boot of the device. You have to check documentation for every model on its own page.

Options

The options you are presented with may vary depending on your device, but there are certain standards that are always there. You can always choose from:

Reboot system now: This will restart the device normally.
Apply update from ADB: The Android Debug Bridge allows you to plug your device into your PC and issue commands from there. It’s designed for developers and requires you to install the Android SDK (software development kit). If you’re interested you can find out more at the Android developer website.
Wipe data/factory reset: This will wipe all of your data (photos, videos, music, apps, everything) and return the phone to factory conditions ready to be setup all over again. It will also wipe the cache partition
Wipe cache partition: This is temporary system data, largely related to app installations and you can delete it without losing any personal data or settings.