Solid State Disks Ltd
The gold standard for NAND remains SLC, where a single bit is recorded in each cell. This gives the highest speed for both read and write along with endurance, defined as the number of erase/write cycles that can be performed.
However this tends to be the most expensive because of the space each bit takes on the chip. MLC stores 2 bits per cell, whilst TLC records 3 bits per cell with the compromise of lower cost for lower performance. 3D NAND stacks layers of either MLC or TLC cells to further increase densities with similar performance compromises.
Various manufacturers then use propriety designs and technology to improve performance to get closer to SLC performance.
DRAM plus NAND
To improve speed and endurance, manufacturers use a DRAM cache in combination with NAND memory arrays.
The algorithms will favour certain applications, such as sequential writes/random reads for logging, or high traffic locations as experienced with a system disks. Our experience can help you select the appropriate manufacturer for your needs.
Manufacturers used DRAM as a cache to improve speed and endurance. The cache may be designed using volatile or SLC NAND. Improving cache hits makes access faster and improves endurance of MLC and TLC NAND, which make up the bulk of the memory.
However, the cache algorithm that is best for sequential write/random read applications such as video or logging, will be different for a system disks. Consideration must also be given to data integrity in the event of power interruption and a disorderly shutdown.
Manufacturers will have various innovations to ensure that there is sufficient time on detection of a falling supply voltage to safely complete the last write command. However, if this is over aggressive this can lead to problems with nuisance shut downs of memory.
The consistent performance of serial interfaces currently outperforms parallel buses.
PATA is a legacy parallel interface used mostly on mature memory products. SD cards have the option of operating with a 4 bit synchronous parallel and can be propriety. USB, SATA, SAS and PCIe are serial interfaces with a varying number of channels or lanes. PCIe ranks highest in performance followed by SAS, SATA and then USB. The NVMe standard is an overlay on PCIe.
With the exception of PATA and some operating modes of SD card formats, all modern high performance memory interfaces are serial using balanced pairs. This has been shown to be highly fault tolerant which allow the data to be clocked at high speeds. USB is a multi-drop serial bus capable of supporting speeds up to 640Mb/s. SATA has separate read and write balanced pairs to form a single half duplex lane with a ATA application layer.
SAS has multiple full duplex lanes supporting a SCSI application layer. PCIe can have up to 16 lanes and data streams are interleaved by bytes between channels. The latest SATA standard and PCIe are usually faster than the fundamental memory technology and therefore not the limiting factor on performance. At SSD we can help inform your choice of memory products to suit your application.
Form factors are available to suit portable application with removable cards such as CF, CFast, USB and SD.
Direct replacements for system drives follow the dimensions of traditional drives from 3.5” to 1.8”. For embedded applications there are a wide variety of formats available to fit the processor board used in your system including BGA packages, PCIe, M.2, DOM and mSATA.