An NVDIMM Primer (Part 2 of 2)

AgigA RamCardTwoThis post is the second of a two-part SSD Guy series outlining the nonvolatile DIMM or NVDIMM.  The first part explained what an NVDIMM is and how they are named.  This second part describes the software used to support NVDIMMs (BIOS, operating system, and processor instructions) and discusses issues of security.

Software Changes

Today’s standard software boots a computer under the assumption that the memory at boot-up contains random bits — this needed to be changed to support NVDIMMs.  The most fundamental of these changes was to the BIOS (Basic I/O Subsystem), the code that “wakes up” the computer.

The BIOS is responsible for detecting all of the computer’s hardware and installing the appropriate drivers, after which it loads the bootstrap program from the mass storage device into the DRAM main memory.  When an NVDIMM is used the BIOS must Continue reading

An NVDIMM Primer (Part 1 of 2)

NVDIMMs are gaining interest lately, so The SSD Guy thought it might be worthwhile to explain both what they are and how NVDIMM nomenclature works.

As I was writing it I noticed that the post got pretty long, so I have split it into two parts.  The first part explains what an NVDIMM is and defines the names for today’s three kinds of NVDIMM.  The second part tells about software changes used to support NVDIMMs in BIOS, operating systems, and even processor instruction sets.  It also discusses the problem of security.

In case the name is unfamiliar, NVDIMM stands for “Nonvolatile Dual-Inline Memory Module.”  Standard computer memory – DRAM – is inserted into the system in the DIMM form factor, but DRAM loses its data when power is removed.  The NVDIMM is nonvolatile, or persistent, so its data remains intact despite a loss of power.  This takes some effort and always costs more for reasons that will be explained shortly.

Although might seem a little odd to discuss memory in a forum devoted to SSDs, which are clearly storage, the NVDIMM is a storage device, so it rightly Continue reading

Comparing Wear Figures on SSDs

DWPD TBW GB/Day TriangleI have been receiving questions lately from people who are puzzled when companies use different parameters than their competitors use to specify the endurance of their SSDs.  How do you compare one against the other?  Some companies even switch from one parameter to another to define the endurance of different SSDs within their product line.

I have found that Intel uses three different endurance measures for its products: DWPD (drive writes per day), TBW (terabytes written), and GB/day.

There’s not any real difference between any of these measures – each one is one way of stating how many times each of the SSD’s locations can be overwritten before the drive has gone past its warrantied life.

The relationships between these three measures are illustrated in this post’s graphic.  You can click on it to see an expanded version.  It’s all pretty simple.  We’ll spell out the relationships in detail below, but in brief, if you want to compare Continue reading

Why 3D XPoint SSDs Will Be Slow

SNIA: Sources of SSD LatencySomething that has been confusing a number of people is the performance of Intel’s 3D XPoint-based SSDs.  Why are they so slow?

Let me back up a little – they’re not really slow.  When Intel compared its standard NAND flash based PCIe SSD to a similar SSD based on 3D XPoint memory, the XPoint model ran 7-8 times faster, which is very impressive.  Intel demonstrated that at the Intel Developer Forum (IDF) last August and several times since then.

But Intel and Micron have been boasting since its introduction that 3D XPoint Memory is 1,000 times as fast as NAND flash.  How do you get from a 1,000 times speed advantage down to a speed improvement of only 7-8 times?

That’s what the graphic in this post will explain.  The small rendition above is just Continue reading