NAND Flash SSD Internals

Solid State Devices (SSD) are complex devices and their performance depends on how they're used. The following description is intended to help software developers understand what is occurring inside the SSD, so that they can come up with better software designs. It should not be thought of as a strictly accurate guide to how SSD hardware really works.

As of this writing, SSDs are generally implemented on top of NAND Flash memory. At a very high level, this media has a few important properties:

  • The media is grouped onto chips called NAND dies and each die can operate in parallel.
  • Flipping a bit is a highly asymmetric process. Flipping it one way is easy, but flipping it back is quite hard.

NAND Flash media is grouped into large units often referred to as erase blocks. The size of an erase block is highly implementation specific, but can be thought of as somewhere between 1MiB and 8MiB. For each erase block, each bit may be written to (i.e. have its bit flipped from 0 to 1) with bit-granularity once. In order to write to the erase block a second time, the entire block must be erased (i.e. all bits in the block are flipped back to 0). This is the asymmetry part from above. Erasing a block causes a measurable amount of wear and each block may only be erased a limited number of times.

SSDs expose an interface to the host system that makes it appear as if the drive is composed of a set of fixed size logical blocks which are usually 512B or 4KiB in size. These blocks are entirely logical constructs of the device firmware and they do not statically map to a location on the backing media. Instead, upon each write to a logical block, a new location on the NAND Flash is selected and written and the mapping of the logical block to its physical location is updated. The algorithm for choosing this location is a key part of overall SSD performance and is often called the flash translation layer or FTL. This algorithm must correctly distribute the blocks to account for wear (called wear-leveling) and spread them across NAND dies to improve total available performance. The simplest model is to group all of the physical media on each die together using an algorithm similar to RAID and then write to that set sequentially. Real SSDs are far more complicated, but this is an excellent simple model for software developers - imagine they are simply logging to a RAID volume and updating an in-memory hash-table.

One consequence of the flash translation layer is that logical blocks do not necessarily correspond to physical locations on the NAND at all times. In fact, there is a command that clears the translation for a block. In NVMe, this command is called deallocate, in SCSI it is called unmap, and in SATA it is called trim. When a user attempts to read a block that doesn't have a mapping to a physical location, drives will do one of two things:

  1. Immediately complete the read request successfully, without performing any data transfer. This is acceptable because the data the drive would return is no more valid than the data already in the user's data buffer.
  2. Return all 0's as the data.

Choice #1 is much more common and performing reads to a fully deallocated device will often show performance far beyond what the drive claims to be capable of precisely because it is not actually transferring any data. Write to all blocks prior to reading them when benchmarking!

As SSDs are written to, the internal log will eventually consume all of the available erase blocks. In order to continue writing, the SSD must free some of them. This process is often called garbage collection. All SSDs reserve some number of erase blocks so that they can guarantee there are free erase blocks available for garbage collection. Garbage collection generally proceeds by:

  1. Selecting a target erase block (a good mental model is that it picks the least recently used erase block)
  2. Walking through each entry in the erase block and determining if it is still a valid logical block.
  3. Moving valid logical blocks by reading them and writing them to a different erase block (i.e. the current head of the log)
  4. Erasing the entire erase block and marking it available for use.

Garbage collection is clearly far more efficient when step #3 can be skipped because the erase block is already empty. There are two ways to make it much more likely that step #3 can be skipped. The first is that SSDs reserve additional erase blocks beyond their reported capacity (called over-provisioning), so that statistically its much more likely that an erase block will not contain valid data. The second is software can write to the blocks on the device in sequential order in a circular pattern, throwing away old data when it is no longer needed. In this case, the software guarantees that the least recently used erase blocks will not contain any valid data that must be moved.

The amount of over-provisioning a device has can dramatically impact the performance on random read and write workloads, if the workload is filling up the entire device. However, the same effect can typically be obtained by simply reserving a given amount of space on the device in software. This understanding is critical to producing consistent benchmarks. In particular, if background garbage collection cannot keep up and the drive must switch to on-demand garbage collection, the latency of writes will increase dramatically. Therefore the internal state of the device must be forced into some known state prior to running benchmarks for consistency. This is usually accomplished by writing to the device sequentially two times, from start to finish. For a highly detailed description of exactly how to force an SSD into a known state for benchmarking see this SNIA Article.