Providing Predictable Performance in Flash and Black-box Storage
Many storage systems are shared by multiple clients with different types of workloads and performance targets. To achieve performance targets without over-provisioning, a system must provide isolation between clients. Throughput-based reservations are challenging due to the mix of workloads and the stateful nature of disk drives, leading to low reservable throughput.
At the same time, virtualization and many other applications such as online analytics and transaction processing often require access to predictable, low-latency storage. Hard-drives have low and unpredictable performance under random workloads, while keeping everything in DRAM, in many cases, is still prohibitively expensive or unnecessary. Solid-state drives offer a balance between performance and cost, and are becoming increasingly popular in storage systems. Unfortunately, SSDs frequently block in the presence of writes, exceeding hard-drive latency and leading to unpredictable performance. Many systems with read/write workloads have low latency requirements or require predictable performance and guarantees. In such cases the performance variance of SSDs becomes a problem for both predictability and raw performance.
First, we present QBox, a new utilization-based approach for black box storage systems that enforces utilization (and, indirectly, throughput) requirements and provides isolation between clients, without specialized low-level I/O scheduling. Our experimental results show that our method provides good isolation and achieves the target utilizations of its clients.
Second, we present Rails, a flash storage system based on redundancy, which provides predictable performance and low latency for reads under read/write workloads by physically separating reads from writes.
More specifically, reads achieve read-only performance while writes perform at least as well as before. We evaluate our design using micro-benchmarks and real traces, illustrating the performance benefits of Rails and read/write separation in flash.
Compared to hard-drives, flash is a more expensive option in terms of raw storage space. We present eRails, a scalable flash storage system on top of Rails that achieves read/write separation using erasure coding without the storage cost of replication. To support an arbitrary number of drives efficiently we describe a design allowing us to scale eRails by constructing overlapping erasure coding groups that preserve read/write separation.