Which file system does Android use? I have read both ext4 and YAFFS2.
Android originally used YAFFS2 as the file system. After Android 2.3, the file system became ext4.
YAFFS2 is usually used for NAND flash in embedded systems such as mobile phones. It includes wear-leveling and a GC mechanism specifically for NAND flash, but it is only single-threaded.
According to an ARS Technica article, the reason that Android switched to ext4 from YAFFS2 for its file system is because YAFFS2 is single-threaded, and "would likely have been a bottleneck on dual-core systems." Android, even back in 2010 was looking forward when many cores could be used in Smartphones. Looking back, it's obvious that this was the right decision. Smartphones went to Dual Core around the time of the Galaxy S2 (2011). More recently most smartphones are released in Quad-Core configuration, with 6 and 8 core configurations on the horizon.