The Nand Flash Cache SSD Cash Dance

Is using flash storage more or less expensive than traditional spinning disks?

Yes, the title for this piece is a play on words. Yet it also reflects the activity in the data storage industry today, particular pertaining to nand flash Solid State Devices (SSD).

For those who need a quick refresher, nand flash is a persistent form of memory, which means that when you remove power, your data stays intact as it would on a hard disk drive (HDD). This differs from dynamic random access memory (DRAM), in that when power is removed, if your data is not saved to disk (e.g. HDD) or other persistent storage (e.g. nand flash SSD), it will disappear.

With that refresher out of the way, let us address something else, which is that nand flash SSD is in your future. In fact, most likely, you are already using it.

For example if you have a smart phone, MP3 player, camera or any device with an SD card (or chip), not to mention a USB thumb drive, you are using nand flash SSDs. Also included in this category are SAS or SATA 3.5” or 2.5” form factor devices with nand flash, also known as electronic flash drive (EFD). Then there are the PCIe cards with nand flash as cache cards, including some HBAs as well as dedicated target devices. And don’t forget DIMM form factor modules that go into servers, storage appliances and systems.

So nand flash is likely in your life, serving purposes ranging from boosting performance speed (read or writing data) as well as its persistency. There is little doubt that nand flash-based SSD solutions are faster reading and writing data than their HDD counterparts.

Thus, vendors will often do all kinds of various dance moves to get you to switch your spending budgets (cash) to acquire more ssd nand flash. Some of the nand flash ssd dance moves including marketing metrics that can be out of this world and lacking context or meaning. For example, a million or more 64 byte sequential read IOPSs might sound impressive, however if your environment is doing 32KB random mixed 70/30 writes vs. reads, all of those small IOPs may not be relevant, hence context matters.

One of the most common mistakes people make is comparing the two different storage mediums (device types) on a cost per capacity basis. This results in SSDs looking more expensive, given their relatively smaller space capacity in the same physical footprint vs. a HDD on a cost basis.

On the other hand, that type of comparison does not take into consideration the difference in SSD speed performance of how fast applications and data can dash when using nand flash vs. HDDs.

Thus, a better comparison is the cost for performance or productivity, such as how many IOPs, or bandwidth per second, or value of response time, transactions, files or videos and web pages or other work done. For example, while a HDD-based solution might have a cost of doing some work of $2.58 per transaction, IOP or other activity, a SSD device could be in the $0.18 range.

Note that those numbers have lots of caveats and your actual mileage, or cash costs, will vary. Your differences will be based on what you use, how it’s configured, and its ability to leverage the work (whether you have the activity to benefit from it), among other factors.

My point is that while on a device-to-device size, and on a “form factor space capacity basis,” a HDD may have a lower cash cost vs. SSD, yet that benefit flip flops when looked at on a performance basis.

AS mentioned above, though, that benefit only exists if you can actually use or have the need for speed. If your applications cannot benefit from being able to dash using flash leveraging the need for speed, then that may not be the optimal solution. However, there are other considerations, including ability to do the same amount of work with fewer devices, removing complexity. Keep in mind that whenever you remove complexity costs are also removed.

Nand flash SSD also come in different variations (besides packaging) including single level cell (SLC), Multi-Level Cell (MLC), enhanced MLC (eMLC) and triple level cell (TLC), among others.

The difference in these in general is that as you add more cells stacked on top of each other your capacity goes up and costs come down. All good things right?

That depends, as there’s a catch. That catch is the more cells stacked on top of each other (e.g. MLC vs. SLC) results in lower write performance (still faster than HDD) along with more wear or reduced durability. This is because while nand flash has no physical mechanical moving parts, since it’s an electronic memory there are in fact cell gates that open and close to store data bits.

Now these gates are not like the mechanical hinged type ones you might be familiar with around your house. Yet these cell gates implemented in silicon (e.g. in the chips) do in fact wear out over time from repeated program/erase (P/E) cycles when data is stored. Note that P/E cycles only occur when data is written or saved, which also includes when data is deleted and garbage collection is performed.

Thus SLC nand flash has a higher cost, however a longer duty cycle so it’s better for write intensive data or as a write cache. MLC has a lower cost with more capacity, yet also lower durability, which is better for read cache or storing data that is seldom updated.

Some vendors and their pundits will dance or cheerlead the hype. There are others who practice the fear dance and that your data will disappear in a flash. So it’s best to find balance and relativity between the hype and fud. Keep things in proper context.

In case you are wondering, what do I use or have? I have nand flash SSDs in my laptop, as well as in my VMware servers that are companions complementing HDD, as well as Hybrid HDD (HHDD) also known as Solid State Hybrid Disks (SSHD). <P>

So to wrap up, nand flash SSD is in your future as it can help your data and applications dash (work faster) including as a cache to traditional HDDs. However, there is a catch, which is the cash, cost. On the other hand, look beyond the cash cost and consider how the nand flash SSD can help you dash, perhaps generating more cash by boosting productivity.

Greg Schulz is Founder and Sr. Analyst of independent IT advisory and consultancy firm Server and StorageIO (StorageIO).