1. UBI – Unsorted Block Images
Artem Bityutskiy

2. Introduction A new technology for flash devices Designed by IBM
Publicly accessible Git tree
GPL license
More information at

3. Plan
Linux MTD overview
UBI overview

4. Linux MTD overview

5. character device (/dev/mtd0)
MTD overview
MTD stands for Memory Technology Devices
MTD is a Linux subsystem (drivers/mtd/)
MTD provides uniform access to various flash devices
MTD provides a generic API for that
MTD provides an “MTD device” abstraction
JFFS2
character device (/dev/mtd0)
MTD device, MTD API
NAND
NOR
DataFlash
AG-AND
OneNAND
ECC’d NOR

6. MTD device MTD device consists of eraseblocks
Eraseblock size varies, typically Kilobytes
Eraseblocks may be written to
, but not re-written
Whole eraseblock has to be erased first
Then it is possible to write there
write
write
write
write
write
re-write
write X
Eraseblock, 128K …
MTD device

7. MDT device vs Block device
Consists of sectors
Sectors are small (512, 1024 bytes)
2 operations: read and write
Bad sectors are hidden by hardware
Sectors do not get worn out
MTD device
Consists of eraseblocks
Eraseblocks are larger ( Kilobytes)
3 operations: read, write and erase
Bad eraseblocks are not hidden
Eraseblocks get worn-out after erasures. 
MTD device is more difficult to handle

8. MTD partitions Flash chip may be split on several MTD partitions
MTD partition is a set of consecutive eraseblocks
MTD partitions is a physical flash area
Partition 1 (e.g., initfs)
Partition 2 (e.g., rootfs)
… so on …

9. Drawbacks of MTD partitions
MTD partitions are static – now way to change them “on-flight”
Do not provide wear-leveling for the whole chip
JFFS2 …
Partition 1
Bad blocks
Partition 2

10. UBI overview

11. Flash File System (e.g., UBIFS)
UBI layer
Flash File System (e.g., UBIFS)
UBI layer
MTD layer …
Physical flash

12. MTD device (physical flash)
Logical Volumes
UBI provides logical volumes instead of MTD partitions
UBI volumes are in a way similar to LVM volumes
UBI volumes may be dynamically created, deleted and re-sized
Volume A
Volume B
Volume B
Volume C
Volume C
Create UBI volume “C” - size 20 logical eraseblocks
Create UBI volume “B” - size 15 logical eraseblocks
Create UBI volume “A” – size 10 logical eraseblocks
Delete UBI volume “A”
Re-size volume “C” to 40 logical eraseblock
… and so on
Re-size volume B to 10 logical eraseblocks
MTD device (physical flash)

13. MTD device (physical flash)
Wear-leveling
UBI does wear-leveling across whole MTD device!
Wear-leveling is done by UBI, not by the UBI user!
JFFS2
Boot volume
RootFS volume
DataStorage volume
MTD device (physical flash)

14. UBI volume vs. MTD partition
Consists of physical eraseblocks (PEB)
Does not implement wear-leveling
Admits of bad PEBs
UBI volume
Consists of logical eraseblocks (LEB)
Implements wear-leveling
Devoid of bad LEBs 
Advantages of UBI
Allows dynamic volume creation, deletion and re-sizing  more flexibility
Eliminates the “wear” problem  simpler software
Eliminates bad eraseblocks problem  simpler software

15. How it works LEBs are mapped to PEBs Any LEB may be mapped to any PEB
erase
read
read
write
Return 0xFFs
Static read-only data
Volume A
Volume B
LEB 0
LEB 1
LEB 2
LEB 3
LEB 4
LEB 0
LEB 1
LEB 2
UBI layer
PEB 0
PEB 1
PEB 2
PEB 3
PEB 4
PEB 5
PEB 6
PEB 6
PEB 7
PEB 8
PEB 9
PEB 10
Low erase counter
High erase counter
Re-map LEB
Move data
MTD device

16. Bad eraseblocks handling
UBI volumes are devoid of bad eraseblocks
UBI does proper error recovery transparently
Write more data
The data have been successfully written!
An UBI volume
Write newcoming data to this PEB
Re-map the LEB to this PEB
Write error! The eraseblock’s become bad!
Partially filled
physical eraseblock
Mark this PEB s bad
Bad physical
eraseblock
Empty physical
Eraseblock
No panic! Recover the data to a good PEB

17. UBI interfaces UBI character devices:
/dev/ubi0, /dev/ubi1, … – UBI devices: volume create, delete, re-size, and get device description operations
/dev/ubi0_0, /dev/ubi0_1, … - UBI volumes: read, write, update, and get volume description operations
UBI sysfs interface: /sys/class/ubi
UBI in-kernel interface: include/linux/mtd/ubi.h

18. Volume update operation
Useful for software updates
Volume is in “corrupted” state if update is interrupted
“Corrupted” volumes are not usable and must be updated

19. Static and dynamic UBI volumes
Dynamic volumes are read-write
Static volumes are read-only
Static volumes are protected by CRC-32 checksum