storage class memoryidke.ruc.edu.cn/seminars/2012/04.13/storage class memory... · 2018. 3. 21. ·...
TRANSCRIPT
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Storage Class Memory: Technology Overview & System Impacts
Zhichao Liang
Phase Change Memory
Storage Class Memory
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Outline
• Why & what is storage class memory?
• A typical storage class memory device: PCM
• The impacts of SCM on database system
• Conclusion
Phase Change Memory
Storage Class Memory
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Outline
• Why & what is storage class memory?
• A typical storage class memory device: PCM
• The impacts of SCM on database system
• Conclusion
Phase Change Memory
Storage Class Memory
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Challenges in building large systems
• Power and cooling
Phase Change Memory
Storage Class Memory
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Challenges in building large systems (cont.)
• Latency gap between CPU and disk
Phase Change Memory
Storage Class Memory
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HDD access time
Phase Change Memory
Storage Class Memory
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HDD access time (cont. )
Phase Change Memory
Storage Class Memory
Access time
rotation time seek time
HDD with a faster than 15,000 rpm almost impossible within the next 10 years!
HDD size will transform from 3.5-inch to 2.4-inch and then 1.8-inch for the best case.
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HDD power
Phase Change Memory
Storage Class 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!
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SSD disadvantage
Phase Change Memory
Storage Class Memory
• The short life time of SSD is an obstacle for its use in industrial-length applications.
• The limited scalability of SSD
hinder its capacity increasing
& price decreasing.
Device Endurance
DRAM 10 power 15
Disk drive 10 power 12
Flash memory 10 power 4 ~ 5
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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)
Phase Change Memory
Storage Class Memory
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Latency gap with SCM
Phase Change Memory
Storage Class Memory
Read/Write PCM (100 – 1000 ns) SCM
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Evolution of memory/storage stack
Phase Change Memory
Storage Class Memory
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Candidate SCM devices
Phase Change Memory
Storage Class Memory
• 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)
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Outline
• Why & what is storage class memory?
• A typical storage class memory device: PCM
• The impacts of SCM on database system
• Conclusion
Phase Change Memory
Storage Class Memory
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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.
Phase Change Memory
Storage Class Memory
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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
Phase Change Memory
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PCM
Slow crystallization affects write performance!
Storage Class Memory
Phase Change Memory
A data cell at each of the intersections of a wordline and a bitline!
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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
Storage Class Memory
Phase Change Memory
An SCM cell at the maximum write times would be worn out within a few minutes! So a wear-leveling layer maybe necessary!
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Taxonomy of PCM system uses
Storage Class Memory
Phase Change Memory
CPU L2 & L3 cache
PCM PCM
control
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:
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Taxonomy of PCM system uses
Storage Class Memory
Phase Change Memory
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
PCM
DRAM
VM contol
PCM control
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Taxonomy of PCM system uses
Storage Class Memory
Phase Change Memory
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.
CPU PCM I/O control
1) Via legacy I/O buses? 2) Via new interface? 3) Page device ?
PCM replaces HDD/SSD completely:
PCM control
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Taxonomy of PCM system uses
Storage Class Memory
Phase Change Memory
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:
PCM DRAM PCM
control HDD I/O control
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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
Phase Change Memory
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Database system overview
Phase Change Memory
Storage Class Memory
Applications
Buffer Pool
Access Methods
Lock
Log
Transaction
HDD
B+ Tree Index Hash Index
Data Page & Log File
LRU, Clock
Read & Write
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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
Phase Change Memory
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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).
• 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!
Asymmetric read & write on PCM
Storage Class Memory
Phase Change Memory
How to reduce write on PCM
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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
Phase Change Memory
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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.
• 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.
Cool and hot data
distinction
Storage Class Memory
Phase Change Memory
Data placement
strategy
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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
Phase Change Memory
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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
Phase Change Memory
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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
Phase Change Memory
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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
Phase Change Memory
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Q & A
Storage Class Memory
Phase Change Memory