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Flash! All-solid-state arrays

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The SSD difference

Solid-state drives combine flash memory and controller electronics to produce a device that operates similarly to a hard drive. Today, there are two types in use: single-level cell (SLC) and multi-level cell (MLC). SLC devices

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store a single bit of data in each cell, while MLC devices are capable of storing multiple (usually two) bits of data with each cell. SLC devices are more expensive per GB, but have a longer lifetime and are typically faster than their MLC counterparts. MLC devices are cheaper per GB, but have issues of endurance (the time before uncorrectable errors start appearing in read/write operations). Some vendors also cite a third category of flash memory, enterprise MLC (eMLC), which is a higher quality version of MLC with better resiliency and longer life.

Startups dominate all-flash array market

The main vendors in the all-flash array space are Kaminario Inc., Texas Memory Systems Inc., Violin Memory Inc. and Whiptail Technologies Inc. at the high end. Expect to pay $15/GB to $20/GB for up to 1 million IOPS per system. In the midrange, Pure Storage Inc. and SolidFire Inc. offer products with 100,000s of IOPS for approximately $5/GB to $10/GB, using data reduction techniques to achieve the lower price point.

Flash memory can be packaged to appear as a hard drive, emulating standard protocols such as SAS and SATA. Most vendors have opted to use these interfaces for their SSD implementations, but while the interface may be the same, SSDs and HDDs have very different characteristics.

Hard drives are mechanical in nature and their moving parts introduce latency into I/O operations. HDDs are particularly poor at handling highly random workloads due to the need to continually move the read/write heads, but they work better with sequential read and write operations.

SSDs have no moving parts and there’s no penalty with either sequential or random I/O, which makes them particularly well suited to certain workload types, such as virtual desktop infrastructure (VDI).

Solid-state drives are an order of magnitude faster than hard drives on both write and read transactions; a typical SSD drive is capable of more than 100,000 IOPS even on random I/O. SSDs are also lighter and use significantly less power than hard disk drives, which can have a bearing on the total cost of ownership (TCO) of storage arrays for many organizations.

This was first published in August 2012

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