What is the /cache partition?

Question

What purpose does the /cache partition in Android serve? Are there any cases where clearing the /cache partition could result in lost data?

Answer 1

/cache partition is mostly for saving the downloads from Google Play Store, found in /cache/downloads. It also can hold the recovery log found in /cache/recovery called last_log as well (this holds any errors and/or diagnostics from the recovery binary a lá CWM or stock, which will get logged in the file to aid debugging and development of the recovery binary).

There is also another directory in there called lost+found which holds any recovered files (if any) as a result of file-system corruption, such as incorrectly removing the SDcard without un-mounting it.

So really, there is no loss of data from clearing it.

In fact, upon Android boot-up, the script will clear out the /cache/downloads to make way for any new downloads/updates from Google Play Store.

Answer 2

To give a closer overview on filesystems used on Android devices, here's a short table with a selection of the more important ones:

+---------------+------------------+------------------------------------------+
| Mount Point   | File System Type | What to find here                        |
+---------------+------------------+------------------------------------------+
| /proc         | proc             | metadata about the system such as file   |
|               |                  | system statistics                        |
| /data/data[1] | YAFFS2           | almost all app data                      |
| /data[2]      | EXT3/EXT4/YAFFS2 | data from apps (except those stored on   |
|               |                  | /data/data) plus system                  |
| /cache        | YAFFS2/EXT3      | Cache file system used by some apps and  |
|               |                  | by the system                            |
| /mnt/asec     | tmpfs            | unencrypted .apk file (the encrypted     |
|               |                  | counterpart is stored in                 |
|               |                  | /mnt/sdcard/.android_secure; decrypted   |
|               |                  | variant is needed for running systems to |
|               |                  | access/execute                           |
| /app-cache    | tmpfs            | temporary file system used e.g. by the   |
|               |                  | stock-browser on some HTC devices        |
| /mnt/sdcard   | vfat             | FAT32 file system on the SD-card         |
| /mnt/emmc     | vfat             | FAT32 file system on the embedded        |
|               |                  | (internal) eMMC                          |
+---------------+------------------+------------------------------------------+

[1] on older systems, entire /data is just a single partition/file system [2] on newer devices, /data can be further segemented

You will not find all these file systems on all devices -- but if you find some of them, this list should give you a raw idea.