1. Solid State Disk Prof. Moinuddin Qureshi Georgia Tech

2. Requirements for Storage Need a non-volatile medium for housing data –Retain data when external power is removed –Needs to keep the data for many years Low Cost –$/GB Current storage systems are Disk based 2

3. Memory Technologies 3 Non-Volatile Volatile Flash Memory

4. Solid State Disk (SSD) Disks that use Flash Memory No moving parts –Less power and heat –Quiet operation –Shock and vibration tolerant Drop-in replacement for disks 4 Image Source: L. Waldock, “Intel X-25M Solid-State Drive”, Reghardware, September 2008.

5. The Architecture of an SSD 5 Source: Agrawal et al., “Design Tradeoffs for SSD Performance”, USENIX 2008

6. Characteristics of Flash Memory Reads/Writes done at the page granularity –Page Size: 2-4 Kilobytes Writes can be done only to pages in the erased state –In-place writes are very inefficient Erases done at a larger block granularity –Block Size: 32-128 pages Time for Page-Read < Page-Program < Block- Erase Limited endurance due to programs and erases These issues are handled by the Flash Translation Layer (FTL) inside the SSD 6

7. Logical Block Map Each write to a logical disk Logical Block Address (LBA) happens to a different physical Flash page –Need a LBA -> Flash page mapping table –Mapping table stored in SSD DRAM and reconstructed when booting –Target flash page for a LBA write is chosen from an allocation pool of free blocks 7

8. Cleaning Writes leave behind blocks with stale copies of the page (superseded pages) –Invoke garbage collection to erase these blocks and add them to the allocation pool page-size copy non- superseded pages in the block to another block before erasure (Write Amplification) 8

9. Cleaning and Wear-Leveling Choose blocks that have the highest number of superseded pages to reduce the number of copies SSD capacity is overprovisioned to mask garbage collection overheads Wear-Leveling –Distribute the program/erase cycles evenly over all the blocks in the SSD 9

10. Challenges For Flash Memory 10 Reliability –Read disturb, write disturb, retention failure Scaling –Becoming very difficult Endurance –Going down exponentially with MLC-2, MLC-3