  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.
