Stochastic Analysis on RAID Reliability for Solid-State Drives

Solid-state drives (SSDs) have been widely deployed in desktops and data centers. However, SSDs suffer from bit errors, and the bit error rate is time dependent since it increases as an SSD wears down. Traditional storage systems mainly use parity-based RAID to provide reliability guarantees by striping redundancy across multiple devices, but the effectiveness of RAID in SSDs remains debatable as parity updates aggravate the wearing and bit error rates of SSDs. In particular, an open problem is that how different parity distributions over multiple devices, such as the even distribution suggested by conventional wisdom, or uneven distributions proposed in recent RAID schemes for SSDs, may influence the reliability of an SSD RAID array. To address this fundamental problem, we propose the first analytical model to quantify the reliability dynamics of an SSD RAID array. Specifically, we develop a "non-homogeneous" continuous time Markov chain model, and derive the transient reliability solution. We validate our model via trace-driven simulations and conduct numerical analysis to provide insights into the reliability dynamics of SSD RAID arrays under different parity distributions and subject to different bit error rates and array configurations. Designers can use our model to decide the appropriate parity distribution based on their reliability requirements.Comment: 12 page

Enhancing SSD Reliability through Efficient RAID Support

A serious problem with current SSDs is its low reliability due to their primary component, flash-memory, that has high error rate and limited erase count. Adopting RAID architecture is a reasonable way to increase reliability of SSDs. In this paper, we propose Dynamic and Variable Size Striping-RAID (DVS-RAID) that dynamically constructs a variable size stripe based on arrival order of write requests such that write requests are sequentially written to a stripe improving the performance and lifetime of SSDs. To increase the reliability of small writes without making use of non-volatile RAM, DVS-RAID employs variable size striping, which constructs a new stripe with data written to portions of a full stripe and writes a parity for that partial stripe. We implement DVS-RAID in the DiskSim SSD extension, and experimental results based on trace-driven simulations show that DVS-RAID out-performs the conventional RAID-5 scheme in terms of performance and lifetime of SSDs