Download - 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