Flash memory is a type of constantly powered nonvolatile memory (NVM) that retains data in the absence of a power supply. Flash memory can be erased and reprogrammed in units of memory called blocks. While the supported filesystems vary based on the device and underlying Linux kernel, the common flash memory filesystems are as follows:

• Extended File Allocation Table (exFAT) is a Microsoft proprietary filesystem that was created to be used on flash drives such as USB memory sticks and SD cards. Because of the license requirements, it isn't part of the standard Linux kernel. But still, a few manufacturers provide support for this filesystem.
• Flash Friendly File System (F2FS) was released in February 2013 to support Samsung devices running the Linux 3.8 kernel. F2FS relies on log-structured methods that optimize the NAND flash memory. The offline support features are a highlight of this filesystem, though it is still transient and being updated.
• Yet Another Flash File System 2 (YAFFS2) is an open source, single-threaded filesystem released in 2002. It is mainly designed to be fast when dealing with the NAND flash. YAFFS2 utilizes Out-of-band (OOB), and this is often not captured or decoded correctly during forensic acquisition, which makes analysis difficult. We will discuss this further in Chapter 9, Android Data Extraction Techniques. YAFFS2 was the most popular release at one point and is still widely used in Android devices. YAFFS2 is a log-structured filesystem. Data integrity is guaranteed, even in the case of a sudden power outage. In 2010, there was an announcement stating that in releases after Gingerbread, devices were going to move from YAFFS2 to fourth extended file system (EXT4). Currently, YAFFS2 is not supported by newer kernel versions, but certain mobile manufacturers might still continue to support it.
• Robust File System (RFS) supports NAND flash memory on Samsung devices. RFS can be summarized as a File Allocation Table 16 (FAT16) or FAT32 filesystem, whereby journaling is enabled through a transaction log. Many users complain that Samsung should stick with EXT4. RFS has been known to have lag times that slow down the features of Android.

Next in line is media-based filesystems.