Understanding UFS LUNs vs eMMC Disks in Android Devices

Understanding UFS LUNs vs eMMC Disks in Android Devices

When working with mobile software, one topic that often confuses technicians is the difference between eMMC and UFS storage, especially when it comes to sectors and LUNs (Logical Unit Numbers). Unlike eMMC, which contains only one main disk, UFS introduces multiple LUNs. This change is not just a small upgrade—it completely reshaped how storage is organized and managed in modern devices.

In this article, we’ll go step by step into why this change happened, how LUNs are structured, and what software technicians should know when dealing with them.

Why eMMC Had One Disk and UFS Has Multiple LUNs

The move from a single-disk system (eMMC) to multiple LUNs (UFS) was necessary for several reasons:

• Speed improvements – UFS is much faster, and dividing data helps parallel operations.
• Isolation & protection – Each cluster of related sectors is separated, which reduces risks if one part is corrupted.
• Flexibility – Easier handling of updates, backups, and recovery.

With eMMC, one disk contained everything. In UFS, however, each LUN works almost like its own mini-disk, each with its own partition table.

Example of LUN Structures in UFS (Qualcomm Devices)

1. LUN0 – Contains the basic and most important partitions:
   • Boot
   • Recovery
   • System
   • FRP (Factory Reset Protection)
   • Userdata
2. LUN1 – Holds Qualcomm’s bootloader segment (XBL).
   • Contains A/B copies (to ensure safety during updates).
   • Includes XBL Config, similar to TrustZone (TEE) in MediaTek.
3. LUN5 – Dedicated to network files such as:
   • Modem and EFS (general and Qualcomm-specific)
   • OEM partitions like oppodynv, opporeserve
4. Last_Parti Sector – Sometimes found after the last sector before SGPT.
• Empty sector used for alignment and size adjustment.
• Helps with stable read/write operations at the end of a LUN.

UFS LUNs in MediaTek Devices

MediaTek devices usually have fewer LUNs, but the concept is similar:

• LUN0 – Contains the first bootloader segment (Preloader).
• LUN1 – Backup of Preloader.
• LUN2 – Contains all sectors (similar to eMMC style).
• W-LUN – Dedicated to RPMB (Replay Protected Memory Block) and Trusted Firmware Environment.
• Access is locked and requires a special processor-generated key.

Why This Matters for Technicians

Let’s say a device has 120 sectors in total. If all of them were grouped under one GPT (like eMMC), then a single corruption could damage the entire disk. With UFS and its LUN-based structure, risks are minimized:

• If the network LUN fails, the bootloader or userdata LUNs are still safe.
• Updates are smoother, since redundant A/B copies are maintained.
• Debugging and repair become easier, as each LUN is isolated.

This division of storage into multiple logical units was a rational and logical choice to meet the demands of modern Android systems, which are constantly updated and prone to crashes or exceptions.

Final Thoughts

In short:

• eMMC = single disk (simpler, older).
• UFS = multiple LUNs (faster, safer, more flexible).

For technicians, understanding these LUNs is essential when flashing, repairing, or analyzing Android devices. Each LUN has its own role, and knowing which one contains what can save a lot of time—and prevent costly mistakes.