Flash memory

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Anne Nicolas

Flash memory — how it works, and how we use it from Linux, SSDs and ‘real’ flash file systems

Technology Business

Flash Memory
David Woodhouse
david@woodhou.se

Kernel Recipes
September 2012

Software and Services Group

Overview

• What is flash?
• What makes flash “special”?
• How to use flash effectively... and safely
• Flash Translation Layers
• Other approaches
• Linux support for flash

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The flash cell
Diagram © Wikipedia user 'Cyferz'
CC BY-SA 3.0

Source Bit
Line Line

Word Line
Control Gate

Floating Gate

N P N

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NOR flash
Diagram © Wikipedia user 'Cyferz'
CC BY-SA 3.0

Bit Line

N N, GND N N, GND N N, GND N
P

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NAND flash
Diagram © Wikipedia user 'Cyferz'
CC BY-SA 3.0

Bit Line

N N N N N N N N N N N
P

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NAND flash: one string

I/O

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NAND flash: one eraseblock

Page

I/O

I/O
16896 bits
per page
I/O
(2048 bytes
+ 64 bytes)

I/O

64 pages per eraseblock

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Data loss on flash

• Charge loss over time
• Charge builds up in insulator
• Write disturb
• Read disturb

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Programming NAND flash
Other Programmed Other
Page Page Page
+10V +20V +10V

+10V I/O

0V I/O

+10V I/O

0V I/O

Programmed cells

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Write disturb
Other Programmed Other
Page Page Page
+10V +20V +10V

+10V I/O

0V I/O

+10V I/O

0V I/O

Disturbed cells

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Read disturb
Other Read Other
Page Page Page
+5V 0V +5V

0V I/O

0V I/O

0V I/O

0V I/O

Disturbed cells

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Single Level Cell

Voltage

Distribution of 0-level cells

Vt – threshold voltage

Distribution of 1-level cells

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Multi Level Cell

Voltage

Distribution of (0,0) cells

Distribution of (1,0) cells

Distribution of (0,1) cells

Distribution of (1,1) cells

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NAND electrical interface

CE RE WE

Address Latch Enable (ALE)
Command Latch Enable (CLE)
NAND
Ready/Busy (R/B) chip

D0-7

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Coping with flash

• Error correction (ECC)
• Bad block management
• Programming pages sequentially
• Garbage collection
• Wear levelling
• Paired pages?

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Flash Translation Layers

1
2
3
4
5
6
7
8

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Flash Translation Layers

1 1
2 2
3 3
4 4
5 chain 5
6 6
7 7
8 8

Write sectors 1-4

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Flash Translation Layers

1 1 1
2 2 2
3 3 3
4 4 4
5 chain 5 chain 5
6 6 6
7 7 7
8 8 8

Write sectors 2, 4, 6

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Flash Translation Layers

1 1 1
2 2 2
3 3 3
4 4 4
5 chain 5 chain 5
6 6 6
7 7 7
8 8 8

“Fold” chain into its final block

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Flash Translation Layers

1 1
2 2
3 3
4 4
5 chain 5
6 6
7 7
8 8

Erase previous blocks in chain

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Flash Translation Layers

1
2
3
4
5
6
7
8

Erase previous blocks in chain

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TRIM

1 1 1
2 2 2
3 3 3
4 4 4
5 chain 5 chain 5
6 6 6
7 7 7
8 8 8

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TRIM

1 1 1
2 2 2
3 3 3
4 4 4
5 chain 5 chain 5
6 6 6
7 7 7
8 8 8

TRIM sectors 1, 3

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TRIM

1 1 1
2 2 2
3 3 3
4 4 4
5 chain 5 chain 5
6 6 6
7 7 7
8 8 8

“Fold” chain into its final block

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TRIM

1 1
2 2
3 3
4 4
5 chain 5
6 6
7 7
8 8

Erase previous blocks in chain

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TRIM

1
2
3
4
5
6
7
8

Erase previous blocks in chain

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An alternative Flash Translation Layer

Sect #1
Sect #2

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An alternative Flash Translation Layer

Sect #1
Sect #2
Sect #1
Sect #2
Sect #3
Sect #4

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An alternative Flash Translation Layer

Sect #1
Sect #2
Sect #1
Sect #2
Sect #3
Sect #4
Sect #2
Sect #4
Sect #6

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File system on top of file system

• Efficiency
– Hard to optimise for opaque lower file system
– Missed garbage collection opportunity
– Real file system knows more about data longevity
– Real file system knows about transactions

• Reliability
– Closed source file system implementation
– No access to underlying medium for fsck / recovery
– Often just implemented badly

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MTD support in Linux

• Synchronous read/write
• Asynchronous erase
• ECC for NAND
• Bad Block management

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UBI

http://www.linux-mtd.infradead.org/doc/ubidesign/ubidesign.pdf

• Static volumes
– Atomically overwritable
– Wear levelled
– CRC protected
• Dynamic volumes
– Writable flash
– Simple logical <-> physical mapping
– Deferred erase (unmap)
– Scrubbing

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Ideal hardware wish list

• Fast queued DMA transfers
• Full error correction in hardware
– With reporting of corrected errors
– Scrubbing
• Page copy

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Questions?

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