I know SSD is supposed to replace storage, so why start picking on DRAM? Because DRAM is an expensive resource that is hard to install in quantity.
The Laptop Case: The number one reason to consider flash SSD in a laptop, as we have seen in our testing, is that it limits the amount of RAM required in that laptop. Meaning that instead of buying an 8-GB laptop you can buy a 2-GB laptop with SSD. Yes, 2 GB of RAM will mean that the operating system has to use virtual memory and swap pages of memory to storage. Now, though, those page swaps will be to a SSD, not a hard disk drive (HDD). In our testing, the performance difference was almost unnoticeable. If you are looking to upgrade the RAM in your laptop, think about upgrading to SSD instead. Get faster drive performance and faster virtual memory.
Flash memory does not always have to be the size of a mechanical hard drive. Using a flash in a memory-module form factor, instead of a drive form factor, can significantly reduce the size of the system. Drive form-factored SSDs, of course, are ideal for upgrades, but new system suppliers should be considering a memory-module approach. This will allow them to build smaller laptops or use the additional space for a larger battery.
The Enterprise Case: While we almost always focus on the performance aspect of flash SSD in the data center, it should also impact how much DRAM you put in a server. Enterprise SSD is faster than what you would put in your laptop--especially PCIe SSD--so thinking of using it as an alternative to additional server memory can be very realistic. The cost of 64 GB or more of DRAM in a server can add up quickly, and it's also risky since it is volatile.
There is also the physical limit of how much DRAM the typical server can physically hold. Comparatively, PCIe flash cards are all in the 500 GB or more capacity range. If you are counting on your database to leverage memory to improve performance, your chances of a cache hit go up dramatically with a 500-GB flash cache vs. a 128-GB DRAM cache. The same is true for a virtualized infrastructure where the need to send pages of memory to storage is more common and the need for RAM more severe.
Where does this leave DRAM? Ironically it has a significant role in storage. As we discussed in our article The Advantages of DRAM SSD, DRAM still enjoys a significant write-performance advantage over flash SSD, as well as a durability advantage. It makes sense then to leverage DRAM as part of a flash solution so that it can take the brunt of the inbound write traffic and increase the life and performance of the overall storage system.
We have always thought of SSD as expensive storage, needed for high-performance situations. Instead maybe (and especially in the server-based case) we should be thinking of it as cheap and more plentiful DRAM and then use a small amount of DRAM where it makes sense in storage systems managing high-write I/O. In fact, mixing the two memory types (flash and DRAM) may become standard practice, especially in the enterprise.
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