Storage Class Memory: Technology Overview & System Impacts

Transcription

1 : Technology Overview & System Impacts Zhichao Liang

2 Outline Why & what is storage class memory? A typical storage class memory device: PCM The impacts of SCM on database system Conclusion

3 Outline Why & what is storage class memory? A typical storage class memory device: PCM The impacts of SCM on database system Conclusion

4 Challenges in building large systems Power and cooling Phase Change Memory

5 Challenges in building large systems (cont.) Latency gap between CPU and disk

6 HDD access time Phase Change Memory

7 HDD access time (cont. ) Access time rotation time HDD with a faster than 15,000 rpm almost impossible within the next 10 years! seek time HDD size will transform from 3.5-inch to 2.4-inch and then 1.8-inch for the best case.

8 HDD power Phase Change Memory 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 their theoretical efficiency limits! S can be improved on smaller size disk drive or shutting down completely when not in use!

9 SSD disadvantage Phase Change Memory The short life time of SSD is an obstacle for its use in industrial-length applications. Device The limited scalability of SSD hinder its capacity increasing & price decreasing. Endurance DRAM 10 power 15 Disk drive 10 power 12 Flash memory 10 power 4 ~ 5

10 Definition of SCM (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) Phase Change Memory

11 Latency gap with SCM Read/Write PCM ( ns) SCM

12 Evolution of memory/storage stack

13 Candidate SCM devices Improved Flash - little change expected in write endurance or speed FeRAM (Ferroelectric RAM) - FeFET MRAM (Magnetic RAM) - Racetrack memory RRAM (Resistive RAM) - Organic & polymer memory PC-RAM (Phase-change RAM) Phace-change RAM (PCM)

14 Outline Why & what is storage class memory? A typical storage class memory device: PCM The impacts of SCM on database system Conclusion

15 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.

16 PCM basic concept Phase Change Memory 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

17 PCM A data cell at each of the intersections of a wordline and a bitline! Slow crystallization affects write performance!

18 PCM vs DRAM Phase Change Memory 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 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!

19 Taxonomy of PCM system uses Memory Oriented: 1) PCM replaces DRAM completely; 2) PCM stands by DRAM. Storage Oriented: 1) PCM replaces HDD/SSD completely; 2) PCM as the cache of HDD/SSD. PCM replaces DRAM completely: CPU L2 & L3 cache PCM control PCM

20 Taxonomy of PCM system uses Memory Oriented: 1) PCM replaces DRAM completely; 2) PCM stands by DRAM. Storage Oriented: 1) PCM replaces HDD/SSD completely; 2) PCM as the cache of HDD/SSD. PCM stands by DRAM: CPU VM contol DRAM PCM control PCM

21 Taxonomy of PCM system uses Memory Oriented: 1) PCM replaces DRAM completely; 2) PCM stands by DRAM. Storage Oriented: 1) PCM replaces HDD/SSD completely; 2) PCM as the cache of HDD/SSD. PCM replaces HDD/SSD completely: CPU I/O control PCM control PCM 1) Via legacy I/O buses? 2) Via new interface? 3) Page device?

22 Taxonomy of PCM system uses Memory Oriented: 1) PCM replaces DRAM completely; 2) PCM stands by DRAM. Storage Oriented: 1) PCM replaces HDD/SSD completely; 2) PCM as the cache of HDD/SSD. PCM as the cache of HDD/SSD: DRAM I/O control PCM control PCM HDD

23 Outline Why & what is storage class memory? A typical storage class memory device: PCM The impacts of PCM on database system Conclusion

24 Database system overview Applications Read & Write B+ Tree Index Hash Index Access Methods Lock Transaction Data Page & Log File LRU, Clock Buffer Pool Log HDD

25 PCM replaces DRAM Phase Change Memory 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!

26 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 Asymmetric buffer pool read write (60ns). How to reduce & 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! write on PCM Rethink of buffer pool replacement strategy!

27 PCM with DRAM Phase Change Memory 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.

28 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. Cool and hot Complete data durability can not be guaranteed but data placement can be improved much. strategy distinction Data Rethink of buffer pool replacement strategy! How to make use of DRAM and PCM to achieve the optimized lifetime of PCM.

29 PCM replaces HDD Phase Change Memory 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.

30 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.

31 PCM with HDD Phase Change Memory 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?

32 Conclusion Phase Change Memory 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.

33 Q & A