There are no simple, definitive rules that would readily identify applications that best suit the EFDs, but we can follow some guidelines. It is very important to understand the load profile of an application before putting it on the EFDs, taking into consideration the fact that most databases have different workload profiles during different times of the day. The EFDs are suitable for highly read intensive and extremely latency sensitive applications and using these drives against the wrong target may not yield the desired benefit for the investment. It is important to understand the following terminology to help with deciding whether the EFDs are suitable for certain workloads.
Write cache: Most of the storage systems have big write side cache and all write IOPS from a host are generally written to cache and incur no delay due to physical disk access. Storage arrays have write caches sized to match the disk count supported by the controller and support enabling and disabling write cache at the LUN level, if needed.
Read hit: A read request from a database host can be served by storage system immediately if it already exists in storage cache because of a recent read or write or due to prefetch. A read serviced from the storage cache without causing disk access is called a read hit. If the requested data is not available in storage cache, the array must retrieve it from disk; this is referred to as a read miss.
Short-stroked drives: Some extremely latency sensitive applications use this technique on regular Fibre Channel drives to obtain low latencies. This is a technique where data is laid out on many partially populated disks in order to reduce the spindle head movement to provide high IOPS at a very low latency.
Workloads with high cache read-hit rates are already serviced at memory access speed, and deploying them on flash drive technology may not show a significant benefit. Workloads with low cache read-hit rates that exhibit random I/O patterns, with small I/O requests of up to 16 KB, and that require high transaction throughput will benefit most from the low latency of EFDs.
Database and application managers can easily point to mission-critical applications that directly improve business revenue and productivity when business transaction throughput is increased, along with reduced service latencies. Cognizant of these applications, storage administrators would often resort to “short stroking” more drives to ensure the highest possible I/O service level supported for them. EFDs can provide two very important benefits for such applications.
A single EFD can replace many short-stroked drives by its ability to provide a very high transaction rate (IOPS). This reduces the total number of drives needed for the application, increases power saving by not having to keep many spinning disks, and may reduce floor space in the data center as well.
EFDs provide very low latency, so applications where predictable low response time is critical and not all the data can be kept at the host or storage cache may benefit from using such drives. Because of the absence of rotating media in EFDs, their transfer rate is extremely high and data can be served much faster than the best response time that can be achieved even with a large number of short-stroked hard drives.
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