Solid State Drive

1. Solid State Drive Feb 15

3. NAND Flash Memory • Main storage component of Solid State Drive (SSD) • USB Drive, cell phone, touch pad…

4. Solid State Drive (nowadays)

5. Solid State Drive (SSD) architecture

6. Comparison

7. Comparison

8. Characteristics of SSD • High reliability (no moving parts) • Small form factor • Less noise • Read/Write latency w.r.t. HDD • No seek latency • Out-of-place update • Invalidate old version, and write new version somewhere else • Invalidated version becomes garbage • HDD, in-place update • Garbage collection (GC) • Erase at the unit of block • LBN to PBN mapping (FTL) • Emulate a block device • Erase cycle limit (endurance) • Read latency 25 us • Write latency 0.25 ms • Erase latency 2.5 ms

9. Advantages of NAND SSD • Fast random read (25 us) • Energy efficiency • High reliability (no moving parts) • Small form factor • Less noise • Widely deployed in high-end laptops • Macbook air, ThinkPad X series, touch pad… • Increasingly deployed in enterprise environment either as a secondary cache or main storage

10. Disadvantages of SSD • Garbage collection (GC) problem of SSD • Stemmed from the out-of-place update characteristics • Update requests invalidate old version of pages and then write new version of these pages to a new place • Copy valid data to somewhere else (increasing number of IOs) • Garbage collection is periodically started to erase victim blocks and copy valid pages to the free blocks (slow erase: 10xW,100xR) • Blocks in the SSD have a limited number of erase cycles • 100,000 for Single Level Chip (SLC), 5,000-10,000 for Multiple Level Chip (MLC), can be as low as 3,000 • May be quickly worn out in enterprise environment • Performance is very unpredictable • Due to unpredictable triggering of the time-consuming GC process

11. Drive read performance

12. Flash Translation Layer (FTL) • Page-mapping • Block-mapping • Log-block-mapping

13. Flash Translation Layer (FTL) • Page-mapping • Mapping any Logical Block Address (LBA) to any Physical Page Address (LPA) • Require huge RAM to store the mapping entries • 512GB SSD requires 4GB RAM to store the mapping table • Each mapping entry is 16 Byte, each page size is 2 KB

14. Flash Translation Layer (FTL) • Block-mapping • Map logical address to fixed offset of the physical address by taking module • LBN%N, N is the number of pages in each block • LBN with the same offset may conflict, high overhead

15. Flash Translation Layer (FTL) • Log-block-mapping

16. Issues about deployment • Mixing read and write degrades the performance • Bandwidth drops as more data is being written • Garbage collection • The performance becomes worse in enterprise environment • Bandwidth can drop to 20% of the bandwidth of the datasheets • Endurance • Wear out in 23 days in enterprise environment • MLC can only wear

17. Mixed writes and reads

18. Improve write performance • OS write caching • Flash specific file system • Drive Write Caching • Multiple concurrent erase blocks • Expensive: 200~400$ /GB

19. Flash File System: • ZFS, has be optimized • JFFS2