Storage Class Memory: Technology Overview & System Impacts
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The document discusses phase change memory (PCM) as a potential storage class memory technology. PCM uses the different resistances of amorphous and crystalline phases of chalcogenide glass to store data. PCM has the potential to bridge the gap between memory and storage by offering non-volatile, solid-state storage that is faster than NAND flash but slower than DRAM. The document analyzes how PCM could impact database systems by replacing DRAM, serving as extended memory paired with DRAM, or replacing hard drives and SSDs. Asymmetric read/write speeds and wear-leveling are challenges to address when using PCM.
Storage Class Memory: Technology Overview & System Impacts
- 1. Phase Change Memory Storage Class Memory: Technology Overview & System Impacts Zhichao Liang frankey0207@gmail.com Storage Class Memory
- 2. Phase Change Memory Outline • Why & what is storage class memory? • A typical storage class memory device: PCM • The impacts of SCM on database system • Conclusion Storage Class Memory
- 3. Phase Change Memory Outline • Why & what is storage class memory? • A typical storage class memory device: PCM • The impacts of SCM on database system • Conclusion Storage Class Memory
- 4. Phase Change Memory Challenges in building large systems • Power and cooling Storage Class Memory
- 5. Phase Change Memory Challenges in building large systems (cont.) • Latency gap between CPU and disk Storage Class Memory
- 6. Phase Change Memory HDD access time Storage Class Memory
- 7. Phase Change Memory HDD access time (cont. ) Access time rotation time seek time HDD with a faster than HDD size will transform 15,000 rpm almost from 3.5-inch to 2.4-inch impossible within the and then 1.8-inch for the next 10 years! best case. Storage Class Memory
- 8. Phase Change Memory HDD power • The power P supplied to a disk drive is given by P = I + M +S. - I : power for interface & control logic; - M: motor to spin the disk; - S : move head to the new track. I & M already close to S can be improved on smaller their theoretical size disk drive or shutting down efficiency limits! completely when not in use! Storage Class Memory
- 9. Phase Change Memory SSD disadvantage • The short life time of SSD is an obstacle for its use in industrial-length applications. Device Endurance DRAM 10 power 15 Disk drive 10 power 12 Flash memory 10 power 4 ~ 5 • The limited scalability of SSD hinder its capacity increasing & price decreasing. Storage Class Memory
- 10. Phase Change Memory Definition of SCM • Storage Class Memory (SCM) is IBM’s term for a new class of data storage and memory devices. • SCM blurs the distinction between Memory (fast, expensive, volatile) and Storage (slow, cheap, non-volatile). • Features of SCM: - Solid state, no moving parts - Short Access times (within an order-of-magnitude of DRAM) - Low cost per bit (DISK like, within an order-of-magnitude) - Non-volatile ( ~ 10 years) Storage Class Memory
- 11. Phase Change Memory Latency gap with SCM Read/Write PCM (100 – 1000 ns) SCM Storage Class Memory
- 12. Phase Change Memory Evolution of memory/storage stack Storage Class Memory
- 13. Phase Change Memory Candidate SCM devices • Improved Flash - little change expected in write endurance or speed • FeRAM (Ferroelectric RAM) - FeFET • MRAM (Magnetic RAM) Phace-change - Racetrack memory RAM (PCM) • RRAM (Resistive RAM) - Organic & polymer memory • PC-RAM (Phase-change RAM) Storage Class Memory
- 14. Phase Change Memory Outline • Why & what is storage class memory? • A typical storage class memory device: PCM • The impacts of SCM on database system • Conclusion Storage Class Memory
- 15. Phase Change Memory Phase change memory • Phase change memory (PCM) is the leading contender for first true SCM. • At least 18 companies are working on PCM, such as Samsung, IBM, Intel, Micro, etc. • PCM is an electronic device using two distinct solid phases of a metal alloy to store a bit. Storage Class Memory
- 16. Phase Change Memory PCM basic concept • Ge-Sb-Te exists in a (quasi)stable amorphous and a stable crystalline phase. - Phases have very different electrical resistances – ratio of 1:100 to 1:1000, and different optical reflectivity. • Transition between phases by controlled heating and cooling - Write ‘1’ (SET) : longer (50ns) weaker current pulse - Write ‘0’ (RESET) : short (10ns) intense current pulse - Read : short weak pulse senses resistance Storage Class Memory
- 17. Phase Change Memory PCM A data cell at each of the intersections of a wordline and a bitline! Slow crystallization affects write performance! Storage Class Memory
- 18. Phase Change Memory PCM vs DRAM Device Type DRAM PCM-S PCM-M Capacity 16GB 128GB 16GB Feature Size 32nm 32nm 32nm Read Latency 60ns 800ns 300ns Write Latency 60ns 1400ns 1400ns Retention Time ms 2-10 years dependent Write Endurance 10 power 15 10 power 8 10 power 12 An SCM cell at the maximum write times would be worn out within a few minutes! So a wear-leveling layer maybe necessary! Storage Class Memory
- 19. Phase Change Memory Taxonomy of PCM system uses Memory Oriented: Storage Oriented: 1) PCM replaces DRAM 1) PCM replaces HDD/SSD completely; completely; 2) PCM stands by DRAM. 2) PCM as the cache of HDD/SSD. PCM replaces DRAM completely: L2 & L3 PCM CPU PCM cache control Storage Class Memory
- 20. Phase Change Memory Taxonomy of PCM system uses Memory Oriented: Storage Oriented: 1) PCM replaces DRAM 1) PCM replaces HDD/SSD completely; completely; 2) PCM stands by DRAM. 2) PCM as the cache of HDD/SSD. PCM stands by DRAM: DRAM CPU VM contol PCM PCM control Storage Class Memory
- 21. Phase Change Memory Taxonomy of PCM system uses Memory Oriented: Storage Oriented: 1) PCM replaces DRAM 1) PCM replaces HDD/SSD completely; completely; 2) PCM stands by DRAM. 2) PCM as the cache of HDD/SSD. PCM replaces HDD/SSD completely: 1) Via legacy I/O buses? CPU I/O control PCM 2) Via new PCM control interface? 3) Page device ? Storage Class Memory
- 22. Phase Change Memory Taxonomy of PCM system uses Memory Oriented: Storage Oriented: 1) PCM replaces DRAM 1) PCM replaces HDD/SSD completely; completely; 2) PCM stands by DRAM. 2) PCM as the cache of HDD/SSD. PCM as the cache of HDD/SSD: PCM DRAM I/O control PCM HDD control Storage Class Memory
- 23. Phase Change Memory Outline • Why & what is storage class memory? • A typical storage class memory device: PCM • The impacts of PCM on database system • Conclusion Storage Class Memory
- 24. Phase Change Memory Database system overview Read & Write Applications Access Methods Transaction Data Page & Log File Lock B+ Tree Index Hash Index Buffer Pool HDD LRU, Clock Log Storage Class Memory
- 25. Phase Change Memory PCM replaces DRAM • PCM buffer pool access (300ns) will be slower than DRAM buffer pool access (60ns). • PCM buffer pool write (1400ns) will be slower than DRAM buffer pool write (60ns). • Data durability can be guaranteed even database server restart or power off. • What about logging? Logging is still necessary? • Wear-leveling can be a problem! • Rethink of buffer pool replacement strategy! Storage Class Memory
- 26. Phase Change Memory PCM replaces DRAM (cont.) • PCM buffer pool access (300ns) will be slower than DRAM buffer pool access (60ns). • PCM buffer pool write (1400ns) will be slower than DRAM buffer pool write (60ns). How to reduce Asymmetric read & write on PCM write on PCM • Data durability can be guaranteed even database server restart or power off. • What about logging? Logging is still necessary? • Wear-leveling can be a problem! • Rethink of buffer pool replacement strategy! Storage Class Memory
- 27. Phase Change Memory PCM with DRAM • Using PCM to extend the DRAM, older data in DRAM will be transmitted to PCM. • Or using PCM and DRAM in parallel mode. • Complete data durability can not be guaranteed but can be improved much. • Rethink of buffer pool replacement strategy! • How to make use of DRAM and PCM to achieve the optimized lifetime of PCM. Storage Class Memory
- 28. Phase Change Memory PCM with DRAM (cont.) • Using PCM to extend the DRAM, older data in DRAM will be transmitted to PCM. • Or using PCM and DRAM in parallel mode. Data Cool and hot • Complete data durability can not beplacement but data guaranteed can be improved much. distinction strategy • Rethink of buffer pool replacement strategy! • How to make use of DRAM and PCM to achieve the optimized lifetime of PCM. Storage Class Memory
- 29. Phase Change Memory PCM replaces HDD • In the near future, the capacity of PCM won’t be as much as disk. • Even if whole DB fits in PCM and even though PCM is persistent, still need to externalize DB regularly, since PCM won’t have good endurance! • In-memory buffer pool can be obviated, or at least read buffer can be obviated. • The read performance of DBMS can be improved much, whatever sequential read or random read. Storage Class Memory
- 30. Phase Change Memory PCM replaces HDD (cont.) • Hardware-enforced block boundaries, such as sector on HDD and page on SSD, are eliminated. - Unlikely large amounts of data become suddenly unavailable - More likely bits, or small ranges of bytes will fail together - Eliminates need for read-modify-write for small updates • Opportunity to rethink data structures for implementing database system, such as B+ Tree, record organization, etc. Storage Class Memory
- 31. Phase Change Memory PCM with HDD • Using PCM to extend HDD, fresh data in PCM & older data in HDD. • Or using PCM for special use, e.g., storing index data on PCM or logging data on PCM. • The write performance can be improved much, whatever sequential write or random write. • How to reduce the amount of write when storing index data on PCM? • The best configuration of PCM & HDD capacity? Storage Class Memory
- 32. Phase Change Memory Conclusion • SCM is an promising technology for building large storage system. • PCM has some special characteristics different from HDD & SSD, which need to be reconsidered for system design. • Research on PCM-based database system just catch the attention of researchers and a lot opportunities are there. Storage Class Memory
- 33. Phase Change Memory Q&A Storage Class Memory