Server and Storage IO Memory: DRAM and nand flash

7.2.2 Memory

Computers rely on some form of memory ranging from internal registers, local on-board processor Level 1 (L1) and Level 2 (L2) caches, random accessible memory (RAM), non-volatile RAM (NVRAM) or nand Flash (SSD) along with external disk storage. Memory, which includes external disk storage, is used for storing operating system software along with associated tools or utilities, application programs and data. Main memory or RAM, also known as dynamic RAM (DRAM) chips, is packaged in different ways with a common form being dual inline memory modules (DIMMs) for notebook or laptop, desktop PC and servers.

RAM main memory on a server is the fastest form of memory, second only to internal processor or chip based registers, L1, L2 or local memory. RAM and processor based memories are volatile and non-persistent in that when power is removed, the contents of memory are lost. As a result, some form of persistent memory is needed to keep programs and data when power is removed. Read only memory (ROM) and NVRAM are both persistent forms of memory in that their contents are not lost when power is removed. The amount of RAM that can be installed into a server will vary with specific architecture implementation and operating software being used. In addition to memory capacity and packaging format, the speed of memory is also important to be able to move data and programs quickly to avoid internal bottlenecks. Memory bandwidth performance increases with the width of the memory bus in bits and frequency in MHz. For example, moving 8 bytes on a 64 bit buss in parallel at the same time at 100MHz provides a theoretical 800MByte/sec speed.

To improve availability and increase the level of persistence, some servers include battery backed up RAM or cache to protect data in the event of a power loss. Another technique to protect memory data on some servers is memory mirroring where twice the amount of memory is installed and divided into two groups. Each group of memory has a copy of data being stored so that in the event of a memory failure beyond those correctable with standard parity and error correction code (ECC) no data is lost. In addition to being fast, RAM based memories are also more expensive and used in smaller quantities compared to external persistent memories such as magnetic hard disk drives, magnetic tape or optical based memory medias.

Memory and Storage Pyramid

The above shows a tiered memory model that may look familiar as the bottom part is often expanded to show tiered storage. At the top of the memory pyramid is high-speed processor memory followed by RAM, ROM, NVRAM and FLASH along with many forms of external memory commonly called storage. More detail about tiered storage is covered in chapter 8 (Data Storage – Data Storage – Disk, Tape, Optical, and Memory). In addition to being slower and lower cost than RAM based memories, disk storage along with NVRAM and FLASH based memory devices are also persistent.

By being persistent, when power is removed, data is retained on the storage or memory device. Also shown in the above figure is that on a relative basis, less energy is used for power storage or memory at the bottom of the pyramid than for upper levels where performance increases. From a PCFE (Power, Cooling, Floor space, Economic) perspective, balancing memory and storage performance, availability, capacity and energy to a given function, quality of service and service level objective for a given cost needs to be kept in perspective and not considering simply the lowest cost for the most amount of memory or storage. In addition to gauging memory on capacity, other metrics include percent used, operating system page faults and page read/write operations along with memory swap activity as well memory errors.

Base 2 versus base 10 numbering systems can account for some storage capacity that appears to “missing” when real storage is compared to what is expected to be seen. Disk drive manufacturers use base 10 (decimal) to count bytes of data while memory chip, server and operating system vendors typically use base 2 (binary) to count bytes of data. This has led to confusion when comparing a disk drive base 10 GB with a chip memory base 2 GB of memory capacity, such as 1,000,000,000 (10^9) bytes versus 1,073,741,824 (2^30) bytes. Nomenclature based on the International System of Units uses MiB, GiB and TiB to denote million, billion and trillion bytes for base 2 numbering with base 10 using MB, TB and GB . Most vendors do document how many bytes, sometimes in both base 2 and base 10, as well as the number of 512 byte sectors supported on their storage devices and storage systems, though it might be in the small print.