SSDs: advantages

SSDs: advantages

• exhibit higher speed than disks• drive down power consumption• offer standard interfaces like HDDs do

SSDs: critical technical constraints

• the absence of in-place update• the absence of random writing on pages• erasure limit : wear out after a certain number of program cycles

Erasure limit: SLC vs MLC

• SLC: 100,000 cycles• MLC: 10,000 cycles

Erasure limit: RBER vs UBER

Solution: SSD-RAID

• RAID offers device-level redundancy• RAID is an effective method of constructing large-scale, high-

performance, and high-reliability storage systems• SSD-RAID combines the advantages of the classic RAID and state-of-

the-art SSDs

Two parity-based SSD-RAID systems

• Differential RAID• CSWL-RAID: Cross-SSD Wear-Leveling• They have a same assumption: parity blocks are updated more often

than data blocks, and devices holding more parity receive more writes and consequently age faster

Differential RAID

• The Problem with RAID for SSDs: • they cause multiple SSDs to wear out at approximately the same rate

Differential RAID: RAID5 case

Differential RAID: features

• Uneven Parity Distribution• Parity-Shifting Drive Replacement

Uneven Parity Distribution: example

• RAID-4: ( 100, 0, 0, 0, 0)• RAID-5: ( 20, 20, 20, 20, 20)• Diff-RAID: ( 40, 15, 15, 15, 15)

Uneven Parity Distribution: aging rate

Parity-Shifting Drive Replacement: example

Parity-Shifting Drive Replacement: example

Analysis of Age Distribution Convergence• Distribution of device ages at replacement time for (80,5,5,5,5) parity

assignment

Analysis of Age Distribution Convergence• Convergent distribution of ages at replacement time for different

parity assignments

Trade-off between reliability and throughput• the more skewed the parity distribution towards a single device• the higher the age differential• the higher the reliability• the lower throughput

Diff-RAID Reliability Evaluation

• Reliability of Diff-RAID• Reliability of Diff-RAID Configurations• Reliability with Different Flash Types• Reliability with Different ECC Levels• Reliability Beyond Erasure Limit• Reliability on Real Workloads

Reliability of Diff-RAID

• Diff-RAID reliability changes over time and converges to a steady state

Reliability of Diff-RAID Configurations

Reliability with Different Flash Types

Reliability with Different ECC Levels

Reliability Beyond Erasure Limit

Reliability on Real Workloads

Diff-RAID Performance Evaluation

• Diff-RAID Throughput• Performance Under Real Workloads• Recovery Time

Diff-RAID Throughput

Performance Under Real Workloads

Recovery Time

Differential RAID: disadvantages

• Assuming a perfectly random workload: without considering the actual age of devices• Parity-Shifting Drive Replacement: the procedure of reconstructing

data and redistributing parity is complex and very time consuming• Trade-off between reliability and throughput: hard to determine a

trade-off point

CSWL-RAID: Why is CSWL needed

• RAID5 and RAID6 cannot ensure wear leveling among devices under a imperfectly random workload

CSWL-RAID: Basic Principle

• change the wearing rate of some SSDs by dynamically adjusting the fraction of parity on them

CSWL-RAID: Practical Architecture

CSWL-RAID: Basic data layout

Age distribution (1,1,1,1)

Age distribution (3,3,3,1)

Age distribution (2,2,1,1)

CSWL-RAID: Improved data layout

Age distribution (1,1,1,1)

Age distribution (3,3,3,1)

Age distribution (2,2,1,1)

CSWL-RAID: Addressing Method

RAID4 case

RAID5 case

Basic CSWL-RAID5 case

CSWL-RAID: Addressing Method• Improved CSWL-RAID5 case

CSWL-RAID: Addressing Method• Improved CSWL-RAID5 case

CSWL-RAID: Average latency

CSWL-RAID: Redistribution time

CSWL-RAID5 case CSWL-RAID6 case

CSWL-RAID: Age difference

CSWL-RAID: Reliability

CSWL-RAID: disadvantages

• All SSDs wear out at approximately the same rate: lower reliability and shorter lifetime• Addressing method is too complex: the complexity of the addressing

algorithm is O(t), where t denotes redistribution times