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. AttributeSSDHDD Random access time0.1 ms5-10 ms Bandwidth100-500 MB/s100 MB/s sequential Price/GB0.9$-2$0.1$ SizeUp to 2TB, 250GB common 4TB Power consumption5 wattsUp to 20 watts Read/write symmetryNoYes NoiseNoYes (spin, rotate)

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