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Why is storage QoS so important for flash arrays?

Storage quality of service (QoS) is important in SSD environments because of resource contention, says Brien Posey in this expert response.

Storage quality of service (QoS) is a technology for guaranteeing a certain amount of storage performance. For example, a database server might require a specific number of IOPS to function efficiently. Storage QoS can maintain the proper performance level for that database server.

Resource contention is the reason why storage QoS is so important in SSD environments. At one time, SSD storage was considered to be so fast that technologies such as storage QoS were unnecessary. Early on, SSD storage had an extremely small capacity, which meant that it was unlikely for multiple workloads to share an SSD unless the SSD was being used solely as a cache.

Today, this is simply not the case. SSDs have steadily increased in capacity over the last few years and, with the advent of all-flash arrays, workload sharing has become the norm. This is especially true in service-provider environments. With multiple workloads competing for IOPS, technologies such as storage QoS are necessary to guarantee bandwidth availability for mission-critical applications.

Another benefit of storage QoS is that it makes storage performance more predictable. Various factors can cause inconsistent performance when using flash storage. One of these factors is sometimes referred to as the noisy neighbor syndrome. In a service-provider environment, for example, it is possible that one tenant could produce enough storage I/O to diminish performance for the other tenants.

Another factor is SSD housekeeping. SSD disks perform automated maintenance features such as wear-leveling and this routine maintenance momentarily impacts performance. Storage QoS can be used to prioritize the available storage bandwidth so that mission-critical applications receive the storage bandwidth that they need even during periods of high storage resource contention.

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