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This slides introduce a novel storage device SCM and its impacts!
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Storage Class Memory: Technology Overview & System Impacts
Zhichao Liang
Phase Change Memory
Storage Class Memory
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
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
Challenges in building large systems
• Power and cooling
Phase Change Memory
Storage Class Memory
Challenges in building large systems (cont.)
• Latency gap between CPU and disk
Phase Change Memory
Storage Class Memory
HDD access time
Phase Change Memory
Storage Class Memory
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.
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!
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
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
Latency gap with SCM
Phase Change Memory
Storage Class Memory
Read/Write PCM (100 – 1000 ns) SCM
Evolution of memory/storage stack
Phase Change Memory
Storage Class Memory
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)
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
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
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
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!
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!
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:
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
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
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
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
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
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
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
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
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
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
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
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
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
Q & A
Storage Class Memory
Phase Change Memory