Solid State Drive (SSD)

  • NAND and NAND type
    NAND is a type of flash memory where information can be stored even when there is no power supplied to the NAND device itself (unlike RAM where data is lost when power is lost) and it can be found in various consumer devices such as Flash/USB/Thumb drives, SSDs and different memory cards.

    In a NAND flash device, bit of information is stored in lot of 'cells'. Depending on the technology used, each cell can store: 1 bit of data (single-level cell or SLC), 2 bits of data (multi-level cell or MLC), 3 bits of data (triple-level cell or TLC) or the coming quad-level cell (QLC) which stores 4 bits of data.

    When it come to performance, it will be slower to read/write from/to a cell with more bit of data compared to a cell with less bit of data. This is because more steps is required to read/write data from/to different bit in a cell. In general, SLC is faster than MLC, is faster than TLC, and finally faster than QLC in this order.
  • Sequential read/write
    In general, sequential read/write indicates how fast a large file can be read/write from the storage device. In my opinion, large file can be any file that's larger than 10MB in size and is 'continuous'. Do not be mistaken for large folder that contains lots of small files such as documents, these folder are large, but the content of the folder is not 'continuous'.

    In term of performance number for SSDs and HDDs, most manufacturers will use this number to indicate how fast their devices are. However, as explained above, this metric only indicate the speed of large file transfer, such as when you want to copy/write movies from/to your device. In practices, many SSDs will have similar sequential read/write number. There will be differences in these number among drives, but usually the differences are insignificant.

    Furthermore, these number doesn't really affect your daily usage that much unless you do a lot of large file transfer. Random read/write numbers will be a more important metric.
  • Random read/write:
    Random read/write indicates how fast/slow it takes to transfer a block of 4KB of data from/to a storage device. In practices, this number can relate to how quick small files (like documents) can be accessed. Most of the times, this metric is presented with an Input/Output Operations Per Second (or IOPS) which means that how many of these 4KB block can be accessed in one second in either read/write operation.

    What makes this metric important is the fact that it's related to how fast a computer can be. Most of computer operation (boot up, shut down, running programs) involve lots of small files access. As such, the higher the random read/write number (especially random read) the faster your computer can boot up etc.
  • Endurance rating:
    Generally refer to the amount of data that can be safely written to the SSD before the life span of the SSD is compromised. Unlike HDD which can be written with infinite amount of data (until the HDD fail of other causes), SSDs performance degrade with the more data written.
  • SLC caching:
    As explained in the NAND type section above, SLC is the fastest NAND type. In order to maintain the high speed of an SSD while reducing cost, manufacturer can have major portion of the NAND of the SSD made from TLC or MLC, and the rest is SLC. When data is written to the SSD, it's first written to the SLC part, which is really fast. Once this SLC part is filled up, the data can be then written to the TLC part, which is a lot slower.

    The implementation of this technique varies from drives to drives, manufacturers from manufacturers. Different drive may have different amount of SLC caching. SLC portion of the drive can be fixed or varied depend on the usage of the drive itself. Once SLC portion filled up, the drive can either write data directly to TLC/MLC part, or the drive can move data from SLC to MLC/TLC part first, then continue writing data to SLC part.

    No matter how this feature is implemented, what the customer will experience in most part is after a certain period of continuous write to the SSD, the speed of SSD may drop. This drop indicates that the drive has exhausted SLC amount, and transitioned to slower write portion of the drive.