In just a few weeks, VxRail will be coming out with a new SATA based SSD options to go along with the new models built on Dell EMC’s new 14G PowerEdge servers. Now, some of you maybe saying, “Wait a minute Nathan. Isn’t that a step back for SSDs?” Well, you aren’t wrong but you’re not completely right, let me explain. With every update of VxRail, it is becoming more flexible in terms of choice, but up until now there has been no choice on what type of SSDs to use. Yet, all of Dell EMCs storage arrays offer many different types of flash drives. SATA SSDs will now give VxRail the ability to offer flash for a much lower price point for customers who don’t need tens of thousands of IOPS but want some of the all flash features in VxRail (i.e. Dedupe, Compression, Erasure coding, etc.). SATA drives are ideal as a low-cost SSD interface where cost, not performance, is the major decision factor.
Now that being said, there are some techinical limitations going with a SATA interface for SSDs. If you are reading this then you may not be familiar with the pros and cons between a SAS and SATA interface for drives, so I will use this post to try to go over some of the differences.
A long time ago in a galaxy far far away, the SATA interface was designed as a hard disk drive (HDD) interface, and the SAS interface was designed as both a device interface and a storage subsystem interface/infrastructure. As HDDs and system requirements evolved (needing faster interfaces and new features) the SATA and SAS interfaces have gone through many versions. The biggest change coming from the throughput requirement that has come with the evolution of solid state drives. SSDs have quickly added significant new performance requirements to these interfaces, as the data rates of SSDs have gone from 10s of MB/sec, to now 1,000s of MB/sec. In addition to the increase in data rates, the lack of mechanical movement in SSDs has also increased the number of IOPS that these storage devices can perform. Hence to move to drives with the Serial Attached SCSI (SAS) interface.
SAS SSD bandwidth options can support speeds from 3 Gbps to 12 Gbps. Meanwhile, the latest incarnation, SATA III, came out in 2009 and can read data at 6 Gbps max (with writes that are bit slower). SAS also delivers less hardware overhead, which is important when latency of data exchange and the number of input/output operations per second (IOPS) becomes an issue. SAS drives also feature more highly configurable reporting structure, and deliver better overall end-to-end data integrity than SATA drives. But the most important difference between SAS and SATA drives is that SAS drives support multiple data paths, which can provide greater data-path redundancy and high availability. That’s the key reason why SAS-based SSDs are generally favored over commodity SATA drives in enterprise settings.
With all this in mind, I am still glad Dell EMC will be offering a new SSD option for customers who are price sensitive, but want the features and performance benefit that comes with the all flash version of VxRail. Especially since some people don’t need tens of thousands of IOPS and will be just fine with a small reduction in the theoretical Max IOPS most clusters can support. Remember, an all flash VxRail will still send all writes to the caching SSD to each node so we are only talking about a small performance reduction in reads.
There will be a lot updates coming to VxRail in the next two months. I will make sure to go over them in depth as they are released.