Elastic SSD Cache with Hybrid RAID Protocols
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Solution Overview
Problem
Existing RAID systems for SSD cache arrays have not adequately addressed the need for improved memory utilization, as they often require unnecessary parity for read data and mirroring for write data, wasting valuable SSD cache memory due to the high premium on memory space.
Innovation Solution
An elastic SSD cache system that combines RAID-0 protocol for read data and RAID-5 single parity protocol for write data, decoupling RAID-5 rows from the physical stripe configuration and using metadata to correlate RAID-5 windows with physical windows, allowing efficient allocation of no-parity read data and single-parity write data in the same cache array.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional RAID protocols with parity and mirroring are used in SSD cache arrays, then data security is improved, but SSD cache memory utilization deteriorates due to the high premium on memory space
Solution Approach 1:
The patent applies different RAID protocols to different data types within the same cache array: RAID-0 (no parity) for read data and RAID-5 (single parity) for write data. This local differentiation allows the system to provide parity protection only where necessary (write data), while avoiding unnecessary parity overhead for read data, thereby improving overall SSD cache memory utilization while maintaining appropriate data security
Solution Approach 2:
The cache array is segmented into distinct regions for read data and write data, each with its own RAID protocol configuration. This segmentation enables independent optimization of each data type's storage approach, allowing the system to maximize cache utilization by applying minimal redundancy to read data and selective redundancy to write data
2Reliability
If RAID-5 single parity protocol is applied to all data in SSD cache, then data reconstruction capability is improved, but cache space efficiency deteriorates due to unnecessary parity for read data
Solution Approach 1:
The patent implements RAID-5 single parity protocol selectively only for write data portions of the cache array, while read data portions use RAID-0 without parity. This local application of parity protection ensures data reconstruction capability is maintained for write data (which has no backup copy), while eliminating unnecessary parity overhead for read data (which already has backup copies stored), thereby optimizing cache space efficiency
3Reliability
If data mirroring is used for write data in SSD cache, then protection against drive loss is improved, but cache space utilization deteriorates due to duplicating write data
Solution Approach 1:
The patent replaces data mirroring with RAID-5 single parity for write data portions of the cache. This substitution reduces the space overhead from 100% (mirroring requires a complete duplicate copy) to approximately 25% (parity requires one parity block per four data blocks in a stripe), thereby significantly improving cache space utilization while maintaining protection against drive loss through parity-based data reconstruction
Data Source
AI summary
The invention provides an elastic or flexible SSD cache utilizing a hybrid RAID protocol combining RAID-0 protocol for read data and RAID-5 single parity protocol for write data in the same cache array. Read data may be stored in window sized allocations using RAID-0 protocol to avoid allocating an entire RAID stripe for read cache data. In the same SSD volume, dirty write data is stored in row allocations using RAID-5 protocol to provide single parity for the dirty write data. Read data is typically stored a window from the physical device having the largest number of available windows. Write data is stored in a row including the next available window in each arm, which decouples the window structure of the rows from the stripe configuration of the physical memory devices.