storage class memory: technology overview & system impacts

Storage Class Memory: Technology Overview & System Impacts

Zhichao Liang

[email protected]

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

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Challenges in building large systems

• Power and cooling

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Challenges in building large systems (cont.)

• Latency gap between CPU and disk

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HDD access time

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HDD access time (cont. )

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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

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• 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

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• 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)

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Latency gap with SCM

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Read/Write PCM (100 – 1000 ns) SCM

Evolution of memory/storage stack

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Candidate SCM devices

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• 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



• The impacts of SCM on database system

• Conclusion

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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.

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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


Phase Change Memory

PCM

Slow crystallization affects write performance!


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


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


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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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PCM stands by DRAM:

CPU

PCM

DRAM

VM contol

PCM control



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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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PCM as the cache of HDD/SSD:

PCM DRAM PCM

control HDD I/O control

Outline



• The impacts of PCM on database system

• Conclusion


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Database system overview

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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!


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!


Asymmetric read & write on PCM


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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.


• How to make use of DRAM and PCM to achieve the optimized lifetime of PCM.


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.


• How to make use of DRAM and PCM to achieve the optimized lifetime of PCM.

Cool and hot data

distinction


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.


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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.


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?


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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.


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Q & A


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storage class memory: technology overview & system impacts

Technology

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