Abstract
Flash-based storage systems offer high performance, robustness, and reliability for embedded applications; however the physical nature of flash memory means that there are limitations to its usage in high reliability applications. In previous work, we have developed RAID architectures and associated controller hardware that increase the reliability and lifespan of these storage systems. However, flash memory needs regular garbage collection and this presents two issues in a high reliability context. The first issue concerns response times as when a garbage collector is active, the flash memory cannot be used by the application layer. This non-determinism in terms of response is problematic in high reliability systems that require real-time guarantees. The second issue concerns lifespan of flash chips. If the garbage collector is allowed free rein over erase operations while garbage collecting, this affects management of the lifespan of each SSD in the array.In this paper we present an enhanced, dynamic, real-time garbage collection method for SSD RAID that does not ignore the strict age distribution management, while offering deterministic response times for access. Real-time efficiency is further improved by dynamically coordinating garbage collection across each device in the array. Our simulation results indicate that the dynamic garbage collection technique maintains the age distribution at a level that does not affect reliability of individual devices. This is evidences using various synthetic and realistic traces dominated by random I/O loads.
Highlights
Flash memory is used as a primary storage medium for embedded systems because of a number of properties including high performance, low power consumption, shock resistance, and small physical size
As probability of read access increases the improvements decrease—10% for 0.3, and 7% for 0.4, and with negligible differences above 0.4 due to garbage collection being invoked much less frequently. These results indicate that the pre-emptive mode controller combined with dynamic garbage collection provides more deterministic response times over PGCRAID, due to the global co-ordination
In this paper we presented enhanced real time garbage collection mechanisms for SSD Redundant Array of Independent Disk (RAID) arrays
Summary
Flash memory is used as a primary storage medium for embedded systems because of a number of properties including high performance, low power consumption, shock resistance, and small physical size. The price of flash memory continues to decrease while density keeps growing with new technologies such as Multi-level cell (MLC) and Triple-level cell (TLC). These technologies increase the density of flash memory, they suffer from lower erase endurance than Single Level Cell (SLC). Error Correction Codes (ECCs)—usually stored in meta data of each page— are used [1]. This technique is insufficient for MLC and TLC devices, and for component failures due to the limited size of the meta data area
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