simg2img, which removes the compression headers and restores the actual filesystem structure before attaching it to a loop device.
Understanding Sparse vs. Raw Images
Android system images, particularly for partitions like userdata, system, and vendor, are typically distributed in a sparse format. This format is designed to save disk space by omitting blocks that contain only zeros. When you inspect the file size of a sparse image, it appears significantly smaller than the actual logical size of the partition defined in the partition table or build configuration.
If you attempt to mount a sparse image directly using standard Linux tools, the operation will fail or result in an unreadable filesystem because the kernel expects a contiguous block of data starting with valid superblock information. The apparent "smaller size" is simply the compressed representation on your host machine's storage. Recognizing this distinction is crucial before proceeding with any mounting operations.
Distinguishing Image Types
You can verify if an image is sparse by checking its header. A raw ext4 image will have a recognizable magic number at offset 0x438 (typically 0xEF53). In contrast, a sparse image has a specific magic number (0xED26FF3A) at the beginning of the file. Tools like file or hexdump can help identify this. If the tool reports the file as "Android sparse image" or shows unexpected bytes at the start, conversion is mandatory.
Converting Sparse Images to Raw Format
The primary step to resolve the mounting issue is converting the sparse image into a raw, uncompressed image. The standard tool for this task on Android development environments is simg2img. This utility reads the sparse format, expands all zero-filled blocks, and writes out a complete, raw binary image that matches the logical size of the partition.
simg2.img input_sparse.img output_raw.imgEnsure that both the input and output paths are accessible and that there is sufficient disk space for the output file. The raw image will be larger than the sparse source but will accurately represent the partition's contents. Once converted, the resulting output_raw.img can be treated like any other standard block device image.
Mounting the Converted Image
After conversion, you need to attach the raw image to a loop device and then mount it. While modern kernels support direct mounting of image files via the -o loop option, explicitly creating a loop device can sometimes provide better control, especially when dealing with large files or specific filesystem quirks.
Using losetup
First, associate the raw image with a free loop device. Use losetup without the -P flag unless you are dealing with a disk image containing multiple partitions. For a single partition image, a simple association is sufficient.
sudo losetup /dev/loop0 output_raw.imgVerify the attachment by running losetup -l to ensure the device is linked correctly. Then, create a mount point directory on your host system.
sudo mkdir -p /mnt/android_userdataFinally, mount the loop device to this directory. Specify the filesystem type if necessary, though auto-detection usually works for ext4.
sudo mount /dev/loop0 /mnt/android_userdataIf the mount fails, check dmesg for errors related to filesystem corruption or mismatched types. In many cases, ensuring the image was fully converted resolves these issues.
Direct Mount Option
Alternatively, you can skip the explicit losetup step and mount the image directly:
sudo mount -o loop output_raw.img /mnt/android_userdataThis method is cleaner for single-use scenarios. Remember to unmount and detach the loop device when finished to free up resources.
sudo umount /mnt/android_userdata
sudo losetup -d /dev/loop0Troubleshooting Common Errors
If you still encounter issues after conversion, consider the following factors. First, ensure you are using a recent version of simg2img from the Android build tools, as older versions may not handle newer sparse formats correctly. Second, check the permissions of the output file; sometimes conversion scripts leave the file owned by root, preventing non-privileged users from accessing it, though sudo mount bypasses this.
Additionally, some Android builds use f2fs or erofs instead of ext4 for the userdata partition. If the default mount command fails, specify the correct filesystem type explicitly, such as -t f2fs. Refer to Understanding Linux Partition Mountpoints for deeper insights into how different filesystems interact with mount options.
Why Not Just Use the Sparse Image?
Attempting to mount the sparse image directly often leads to confusion because the kernel cannot parse the sparse header as a valid filesystem superblock. The "smaller size" is a symptom of this compression. By converting to raw, you align the physical file structure with the logical expectations of the Linux VFS (Virtual File System). This approach is consistent with how Android recovery modes and fastboot utilities handle these images internally.
For more complex scenarios involving external drives or fstab configurations, see Mounting External Drives with mount command in fstab not working: A Troubleshooting Guide to understand broader mounting principles that apply here as well.
Frequently Asked Questions
Can I mount a sparse image without converting it?
No, standard Linux mount commands cannot interpret Android sparse image headers. You must use simg2img to create a raw equivalent first. Some specialized Android tools might handle this internally, but general-purpose OS utilities require the raw format.
What if simg2img is not installed on my system?
You can obtain simg2img from the Android SDK platform-tools package or by building it from the Android source code under system/extras/simctool. On Ubuntu, it may be available via apt install android-sdk-platform-tools-common.
Why does my mounted userdata partition look empty?
This is normal for fresh images. The userdata partition is formatted but contains no user data until the Android system boots and populates it. If you expect pre-loaded apps, ensure you are looking at the correct partition, such as product or system_ext.
Is it safe to delete the raw image after mounting?
Yes, once you have mounted the image and verified its contents, you can safely delete the raw image file to reclaim disk space. The sparse original should also be kept as a backup since it is much smaller and suitable for distribution.
How do I check the filesystem type of an Android partition?
Check the build configuration files (like BoardConfig.mk) or use the file command on the raw image after conversion. It will typically report ext4, f2fs, or erofs depending on the Android version and device target.