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Tuning SSD RAID for optimal performance

Flash Stack

Article from ADMIN 28/2015

By Werner Fischer , By Georg Schönberger

Hardware RAID controllers are optimized for the I/O characteristics of hard disks; however, the different characteristics of SSDs require optimized RAID controllers and RAID settings.

Conventional hard disks store their data on one or more magnetic disks, which are written to and read by read/write heads. In contrast, SSDs do not use mechanical components but store data in flash memory cells. Single-level cell (SLC) chips store 1 bit, multilevel cell (MLC) chips 2 bits, and triple-level cell (TLC) chips 3 bits per memory cell. Multiple memory cells are organized in a flash chip to form a page (e.g., 8KB). Several pages then form a block (~2MB).

At this level, the first peculiarity of flash memory already comes to light: Whereas new data can be written to unused pages, subsequent changes are not possible. This only works after the SSD controller has deleted the entire associated block. Thus, a sufficient number of unused pages must be available. SSDs have additional memory cells (spare areas), and depending on the SSD, the size of the spare area is between 7 and 78 percent of the rated capacity.

One way of telling the SSD which data fields are no longer used and can therefore be deleted is with a Trim (or TRIM) function. The operating system tells the SSD controller which data fields can be deleted. Trim is easy to implement for a single SSD, but for parity RAID, the implementation would be quite complex. Thus far, no hardware RAID controller supports Trim functionality. This shortcoming can be easily worked around, however: Most enterprise SSDs natively come with a comparatively large spare area, which is why Trim support hardly matters. And, if the performance is not good enough, you use overprovisioning – more on that later.

Metrics

When measuring SSD performance, three metrics are crucial: input/output operations per second (IOPS), latency, and throughput. The size of an I/O operation is 4KB unless otherwise stated; IOPS are typically "random" (i.e., measured with randomly distributed access to acquire the worst-case values). Whereas hard drives

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