The document discusses solid state drives (SSDs) and how they compare to traditional hard disk drives (HDDs). It notes that SSDs offer advantages over HDDs in terms of performance, power consumption, reliability, and environmental ruggedness. However, SSD endurance and lifetime are affected by factors like write amplification, wear leveling efficiency, and capacity. Larger SSD capacities and advances in controlling write amplification are extending SSD lifetimes to meet consumer needs.

1. SSDs
A Shift in Data Storage
Todd Dinkelman
Micron Technology, Inc.
Santa Clara, CA USA
August 2008 1

2. Agenda
SSD Defined
Quick Comparison
Why MLC
Reliability/Endurance
Summary
Santa Clara, CA USA
August 2008 2

3. What is Being Called an SSD?
USB interface embedded solution
Module-based
IDE interface card w/o enclosure
1.8-inch and 2.5-inch, 32GB and 64GB SSD
for notebook and performance computing
Module/card with custom form factor, density,
and interface
Santa Clara, CA USA
August 2008 3

4. The Complexity of HDDs
Cover mounting holes
Base casting
HDD Advantages
• Density
Spindle
• Price/GB
Slider (and head)
Actuator
arm
Platters
Actuator
Flex circuit
SATA interface (attaches heads
connector to logic board)
Power connector
Actuator axis
Source: Panther Products
Santa Clara, CA USA
August 2008 4

5. The Simplicity of SSDs
SATA interface
SSD Advantages
connector
Performance
•
Size
•
Weight
•
Ruggedness
•
Temperature
•
Range
• Power
NAND
Printed Circuit
Controller
Santa Clara, CA USA
August 2008 5

6. SSDs vs. HDDs
Due to recent advances
SSDs HDDs in NAND lithography,
SSD densities have
Capacity
reached capacities for
Performance mass market appeal
Reliability
SSDs offer many
Endurance features that lead to
improved user
Power
experiences
Size
Weight Early shortcomings
regarding reliability and
Shock
endurance are being
Temperature overcome
Cost per bit
Santa Clara, CA USA
August 2008 6

7. SSDs in Computing
CPU
Relative Relative
Latency Cost/Bit
1 L1 Cache 1,800
2.5 L2 Cache 1,400
1,200 DRAM 10
NAND 25,000 SSD 3
25,000,000 HDD 1
NAND Flash closes the latency gap
Santa Clara, CA USA Cost/bit data as of November 2007
August 2008 7

8. NAND in Computer Architecture
CPU
DRAM
Memory
DRAM
DDR3 PCIe Graphics
& Bus
Control
Link
Peripheral
USB NVMHCI
PCIe
USB & Bus
Flash Drive Flash
Control
SATA
Hybrid
SSD
HDD
Santa Clara, CA USA
August 2008 8

9. MLC and SLC Differences
SLC
Single-level cell
•
– One bit per cell
MLC
Multi-level cell
•
– 2 bits per cell – today
– 3 and 4 bits per cell – future
Endurance
SLC is typically 10 times better than MLC
•
Performance
SLC provides ~2X the write performance of MLC
•
Price
SLC-based products have better than 2X the
•
price/GB compared to MLC
Santa Clara, CA USA
August 2008 9

10. SSD Market Trends
Improvements in controller technology
• Moving from CompactFlash architectures to true
SSD controllers
Notebooks and PCs
• Migration to MLC
– Light usage model
– Cost, size, and performance are all important
• Value Proposition
– Better than desktop performance in an ultra-light
notebook
Santa Clara, CA USA
August 2008 10

11. SSD Average Price/GB
1.8quot; HDD
MLC SSD
$/GB
2004 2006 2008 2010 2012 2014
Santa Clara, CA USA
August 2008 11

12. NAND Reliability and Endurance
Effect Description Observed As Management
Cells not being
programmed Increased read
Program ECC and Block
receive charge errors immediately
Disturb Management
via elevated after programming
voltage stress
Cells not being
Reliability read receive Increased read
Read ECC and Block
charge via error at high
Management
Disturb elevated voltage number of reads
stress
Data Charge loss over Increased read ECC and Block
Retention time errors with time Management
Cycles cause Failed
Endurance/
Endurance charge trapped in program/erase Retire Block
Cycling dielectric status
Santa Clara, CA USA
August 2008 12

13. NAND Error Rate
Bit Error Rate
– Failing bits corrected with appropriate levels of ECC
– Correctable bit errors do not result in data loss
Raw Bit Error Rate (RBER): Bit error rate
prior to ECC
Uncorrectable Bit Error Rate (UBER): Bit
error rate after ECC
UBER is projected using the measured RBER
and specific level of ECC
Santa Clara, CA USA
August 2008 13

14. SSD Reliability and Endurance
SSD reliability has two parts
• MTBF
– Measure of time between failures due to manufacturing or
component defects
– 2 million hours is typical for Micron SSDs
• Endurance
– SSDs all wear out due to data writes
– Indication of drive life based on a usage condition
– Micron SSDs are specified to last 5 years under predefined
usage conditions
– Usage conditions vary for consumer and performance products
Santa Clara, CA USA
August 2008 14

15. Endurance Factors
Wear-leveling efficiency
Write amplification
NAND cycles
SSD densities
Santa Clara, CA USA
August 2008 15

16. Wear-Leveling
Dynamic wear-leveling uses the available
free space and the incoming data to equally
wear each of the physical blocks
• Static data is not included in the available pool of
wear-leveling blocks, leaving a portion of the drive
with no wear
Static wear-leveling considers all physical
blocks in the SSD, regardless of content, and
maintains an even level of wear across the
entire drive
Santa Clara, CA USA
August 2008 16

17. Wear-Leveling Example
MLC devices can typically support 10,000 cycles per
block
If you erased and reprogrammed one block every
second, you would exceed the 10,000 cycling limit in
just 3 hours!
60 x 60 x 3 = 10,080
Rather than cycling the same block, wear-leveling
involves distributing the number of blocks that are
cycled
Santa Clara, CA USA
August 2008 17

18. Wear-Leveling Example (continued)
An 8GB MLC-based SSD contains 32,768 independent
blocks (each block is 256KB of data)
If we took the previous example and distributed the cycles
over all 32,768 blocks, each block would have been
programmed once after 9 hours
If you provided perfect wear-leveling on an 8GB drive, you
could erase and program a block every second, every day
for over 10 years!
10,000 X 32,768 327,680,000
= = 3,792 days = 10.38 years
60 X 60 X 24 86,400
Santa Clara, CA USA
August 2008 18

19. Write Amplification
Additional write overhead due to garbage collection
and wear-leveling
• Write Amplification = Flash Writes / Host Writes
Influence on host data patterns
• Large number of small random transfers increases write
amplification
Influenced by number of spare blocks and static data
• Keeping a percentage of the drive as spares will bound the
write amplification
Santa Clara, CA USA
August 2008 19

20. Typical PC File Transfers
Avg. Disk Bytes Per Read/Write
1800
1600
1400
1200
1000
800
Reads
Writes
600
400
200
0
Less than 4KB to 16KB to 32KB to 64KB to 128KB to Greater
4KB 16KB 32KB 64KB 128KB 256KB than
256KB
Transfer Size
Santa Clara, CA USA
August 2008 20

21. SSD Endurance Calculation
Write amplification and wear-leveling
efficiency must be accounted for when
calculating SSD lifetime
NAND Cycles * SSD Capacity
Life in Years =
Amplification Factor * GB/Year
Santa Clara, CA USA
August 2008 21

22. SSD Lifetime
Given a capacity point, NAND endurance, and writes-per-day, the
upper limit on SSD lifetime is defined
Example: A 64GB SSD, 10K PROGRAM/ERASE cycles and write duty of
•
350 GB/day will wear out in 5 years
Uneven wear within a NAND Flash device may cause ‘hot spots’
Wear-leveling of NAND is necessary
•
Improper NAND data management may cause lifetime to be
significantly less than the limit
A Flash ERASE operation must occur before a page can be rewritten
•
To improve overall throughput, erase granularity is much larger than write
•
granularity
LBA traffic pattern can cause lots of extra data movement
•
Santa Clara, CA USA
August 2008 22

23. SSD Endurance
Average Number Of Times Per Day The Drive Space Gets Written
60
33% WRITEs
50
2K random IOPs
Times per Day (Count)
40
64GB drive
(55GB user-accessible
30
20% reserved spares
20
10
0
0 12 24 36 48 60 72 84 96
Required Lifetime (Months)
Santa Clara, CA USA
August 2008 23

24. SSD Capacity Effect on Endurance
Years for SSD to Wear Out
Time to Failure
32 64 128 256 512
Drive Capacity (GB)
SSD life will double with every doubling of capacity
Santa Clara, CA USA
August 2008 24

25. Summary
SSDs bring many advantages to storage
Performance
•
Power
•
Reliability
•
Environmental ruggedness
•
Different products for different markets
• MLC-based SSDs for PCs
Reliability has two aspects
• Endurance and MTBF
Santa Clara, CA USA
August 2008 25

26. Thank You
Santa Clara, CA USA
August 2008 26