1. Design Tradeoffs for SSD Performance
N. Agrawal, V. Prabhakaran, T. Wobber, John D. Davis, M. Manasse, R. Panigrahy.
Microsoft Research, Silicon Valley,
Usenix ’08, ATC, 2008
Thank You!
Presentation by Lee, Jeyeon

2. Introduction
Background
SSD Basics
Evaluation
Conclusion

3. 4 issues relevant to SSD performance
1. Introduction
4 issues relevant to SSD performance
Data placement
placement of data is critical to provide load balancing and wear-leveling.
Parallelism
memory components must be coordinated so as to operate in parallel.
Write ordering
small, ramdomly-ordered writes are tricky
Workload management
This presents a taxonomy of design with a trace-driven simulator extracted from real systems.

4. 2. Background
Flash Internals
SSD
Flash Package (4 GB)

5. Properties of Flash Memory
2. Background
Properties of Flash Memory
read : 25μs + 25ns/byte
write : 100μs + 200μs
erase : 1.5ms
constraints : erase before reuse, erase limit(≈100K)
Bandwidth and Interleaving
there is bottleneck in SSD performance  “Interleaving”
32MB/sec  40MB/sec
13MB/sec  26MB/sec

6. 3. SSD Basics
Logical Block Map
writes cannot be performed in place as on a rotating disk.
 each write of a single LBA corresponds to a write of a different flash page.
 SSD must maintain mapping between LBA and physical flash location.
writes must be performed sequentially whenever possible.
If a contiguous range of LBAs is mapped to the same physical die,
performance for sequential access in large chunks will suffer. 1 2 3 4 5 6 7 1 2 3 4 5 6 7
LBA
FTL
FTL 4 1
Physical Block 1 5 2 3 2 6 4 5 3 7 6 7

7. 3. SSD Basics
Cleaning
When a page is written, previous mapped contents are now out-of-date.
We need a garbage collector to allocate fresh blocks.
overprovisioning

8. Parallelism and Interconnect Density
3. SSD Basics
Parallelism and Interconnect Density
To achieve bandwidths, I/O requests are should handled in parallel.
Parallel requests
Ganging
Interleaving
Background cleaning
Persistence
Flash memory is by definition persistent storage.
Rebuilding the logical block map and all related data structures is essential.
Error detection and correction must be provided by application firmware.

9. Wear-leveling Greedy Rate limiting Migration
3. SSD Basics
Wear-leveling
Greedy
recycle when a candidate’s remaining lifetime exceeds the threshold.
Rate limiting
rate-limit worn out blocks’ usage
Migration
migrate cold data into old blocks
< Block Wear in IOzone >
< Lifetime distribution >

10. Microbench, Interleaving
4. Evaluation
Microbench, Interleaving
Ganging Performance

11. 5. Conclusions
The NAND-flash based SSD is certain to represent a sea change in the architecture of computer storage subsystems.
Many of the issues that arise in SSD design appear to mimic problems that have previously appeared in the storage stack.
Trace-driven simulator and workload traces extracted from real systems will presents a taxonomy of such design choices.

12. Design Tradeoffs for SSD Performance
N. Agrawal, V. Prabhakaran, T. Wobber, John D. Davis, M. Manasse, R. Panigrahy.
Microsoft Research, Silicon Valley,
Usenix ’08, ATC, 2008
Thank You!
Presentation by Lee, Jeyeon