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Who needs Latency ?

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In simple terms, latency is the amount of time it takes to complete a given task. The most common example everyone is aware of is about application response time i.e. when you are using an application and request information to be processed or provided, latency is the term used for the amount of time it takes for the application to request the data and give you a response. Several major hyper-scale vendors have publicly stated that Latency matters. Amazon has reported that every 100ms of latency cost them 1% in sales. No wonder service level agreements for application owners often explicit spell out latency requirements for acceptable end user performance. Storage plays a critical role because the information being processed and requested is typically persisted on some form of non-volatile storage – this is true for both hard drives and SSDs, and the amount of time can differ for a variety of reasons. The table below highlights typical latency times associated with particular device types. I’ve tried to represent these speeds/feeds in a more meaningful “Human” scale form.

Device	Latency	Human Scale
Srv RAM	100ns	1 sec
Flash Dimm	2.5 – 5 microsecs	25-30 secs
PCI-e SSD	20 -50 microsecs	3 – 8 mins
SATA / SAS drives	100 – 300 microsecs	16 – 50 mins
All Flash Arrays	500 – 1100 microsecs	1.5 – 3 hrs
Hybrid Arrays	2 millsecs	6 hrs
15k RPM Drive	5 – 15 millsecs	15hrs

When it comes to SSDs, however, latency can still become a factor, varying significantly based on the interface and where the flash sits in the infrastructure. For instance, flash-based devices using the SAS and SATA interfaces have an order of magnitude higher latency that a PCIe drive does. For some applications, having this much lag within the system isn’t a problem. However, for virtualised environments or cloud computing, that lag time can quickly bring things to a crawl impacting the end users’ experience. In those scenarios PCIe works better. That being said, even 20-50 microsecond latency can be too much, e.g. Big Data analytics, where a few seconds of lag time can mean millions of dollars in lost revenue. . Another item that comes into play due to the interface is latency consistency. SAS and SATA devices have multiple gates they must go through in order to go through their complete input/output operation, specifically the I/O hub. Because there are only so many lanes contention is introduced by the I/O hub, causing data to build up and make its way through more slowly. This leads to latency peaks and valleys. PCIe devices avoid the I/O hub, however have a PCIe hub introduced into the data path, which can lead to some spikes as well. Server Side Caching solutions, which utilize server memory (RAM) remove both of these points of contention,  resulting in a much more consistent, horizontal latency output. Reference :

1. Howard Marks of DeepStorage.net

2. Hemant Gaidhani – Director, Product Marketing and Management, SanDisk Enterprise Storage Solutions