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What Is The Lifespan of a USB Flash Drive?

The lifespan of a USB flash drive relates to three factors. In general terms, a flash drive will last much longer than you think and here are some details to help you understand the answer.

The three factors related to the life span of a USB flash drive are:

  • How the drive is made
  • Wear leveling technology
  • How the drive is treated

Flash drives are a commodity product and (generally) driven by lowest price. With that in mind there are plenty of shortcuts a manufacturer can use to save time and money. What is important to understand, is knowing the quality of product you are going to use.

How the drive is made

A flash drive is made up of five primary components: The PCB (printed circuit board) the flash memory, the USB controller, the components and the soldering which holds everything together.

Printed Circuit Board (PCB)

Most promotional memory products (flash drives given away at trade shows) will use a two layer printed circuit board. Two layer boards are bad for use with any USB device, including a flash drive. The USB specification requires four layers for a product to be made to specification. A four layer board will include the, much needed, grounding plane of the PCB to insure transmission without interference from the trace lines. A two layer board is at a much greater risk of not performing as it should. If you received a USB at a trade show, don’t consider that device for “long term” or “important” storage options.

This is an image of a four layer USB flash drive by Nexcopy with Micron memory with write speeds of 12MB/s

USB flash drive PCB with NAND memory

Flash Memory

Flash memory used in the production of USB drives stems from a sea of unknown factors. Flash drives are the bottom of the barrel when it comes to NAND memory as part of the BOM (bill of material). All the good quality NAND memory is used for more expensive products like phones, set-top boxes, communications hardware, etc… and the manufacturer of USB flash drives, is typically, the last tier of manufacturing consumption. With that in mind, one trick the flash manufacturers use, is down-sizing the memory wafer (NAND chip). Let us provide an example: Toshiba is the world’s largest memory manufacturer, and after production of say a 64GB chip, they test it. If the quality of the silicon cells in the chip are below a certain percentage, the chip gets downgraded to a 32GB chip. They test it. If the memory is still failing QC, it gets downgraded again. The process continues. So if you are dealing with a 512MB USB stick, you are dealing with the worst part of NAND memory chip. Very unreliable. The quickest way to test the quality of flash memory is test the write speed. For USB 2.0 product, if you see a write speed of 9-10MB/second or better… its’ good quality. For USB 3.0 if you see a write speed of 18-20MB/second or better you are dealing with good quality. A slow write speed means the silicon of the chip is having a harder time making the phase change (positive or negative) to save data to the memory chip.

USB Controller

The USB controller is the chip on the flash drive with all the brains. The USB controller is the gate keeper between the host computer and the USB stick. The chip allows the host computer to read or write data to and from the flash memory on the flash drive. Because the USB controller is the brain of the flash drive, it’s important to have a controller that performs well and is reliable. One of the most important features of the USB controller is wear leveling. This is also one of the most important aspects for defining the lifespan of a USB flash drive. More about wear leveling in a bit. For now, the important point is understanding the compatibility of the USB controller to that of the flash memory. The NAND memory market is very fast pasted. New technology is always developing. For this reason, the firmware inside the USB controller is very important. The firmware “marries” the flash memory to the device and creates a usable flash drive. There are many flavors of firmware for a single controller and it all boils down to how often the USB manufacturer updates those firmware tools. It is very possible to load firmware that is not optimized for the NAND memory used in the production of the flash drive. It’s also very possible the firmware is set for a different objective, for example, the firmware was set to be optimized for capacity rather than read/write speeds. The amount of control the USB factory has with these firmware tools is mind-blowing. The firmware tools are used to configure the USB stick to exactly what they want. In summary, there is no real way to test the quality of the USB controller and it’s firmware other than having an intimate knowledge and relationship with the actual manufacturer of the USB flash drive. The point to explaining the function of a USB controller is to show what a large impact it has on the overall performance of the drive.

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Longevity of USB Flash and Wear Leveling

I think we have all heard a USB can only be used so many times.  Some say the number is 1,000 writes – some say the number is 100,000 writes.  One thing I do know for sure, it’s impossible to tell on any one specific device.  The life cycle of a USB is directly related to the flash memory…and from model to model or style to style, who knows what quality of flash is used.  With that said, we can still explain the theory behind making USB drives last longer.  For the most part it boils down to several elements  A)  the memory type and quality and B) the wear leveling technique. As a quick summary the NAND flash in USB can be either SLC, MLC or TLC (single cell, multi-layer cell or triple-layer cell memory).  Typically you will find MLC and now mostly TLC in USB sticks.  SLC can be found but typically on the very high end devices. Wear leveling is a technique to prolong the life of the erasable flash memory.  To summarize, flash memory has individual, erasable segments that can be set as zero’s or ones (set as either positive or negative charge).  However, after a certain number of erase and write cycles the segment (cell) becomes too unstable for reliable use. Wear leveling is the algorithm used by the controller on the device which attempts to arrange the erase and writes evenly across the flash medium.  Typically flash can have a cycle between 3,000 and 5,000 erase/writes.  In addition to the usable area, the flash also has some cells with specific blocks for extended live which can handle up to 100,000 writes.  This is the area where the controller makes not of the segments previously used and maps out the next best cells to use during an erase/write cycle. There are three types of wear leveling. No wear leveling – A Flash memory storage system with no wear leveling will not last very long if it is writing data to the flash. Without wear leveling, the Flash controller must permanently assign the logical addresses from the host computer to the physical addresses of the Flash memory. This means that every write to a previously written block must first be read, erased, modified, and re-written to the same location. This is very time consuming and highly written locations will wear out quickly with other locations even being completely unused. Once a few blocks reach their end of life the drive is no longer operable. Dynamic wear leveling – The first developed type of leveling is called dynamic wear leveling and it uses a map to linklogicl block addresses from the host to the physical Flash memory. Each time the host writes replacement data, the map is updated so the original physical block is marked as invalid data, and a new block is linked to that map entry. Each time a block of data is re-written to the Flash memory it is written to a new location. Static wear leveling – The other type of wear leveling is called static wear leveling which also uses a map to link the block addresses to physical memory addresses. Static wear leveling works the same as dynamic wear leveling except the static blocks that do not change are periodically moved so that these low usage cells are able to be used by other data. This rotational effect of block addressing enables an SSD to operate until most of the blocks are near their end of life. The above are three types of wear leveling and there are three types of techniques used to extend the life of a USB drive. Error correction – Code which is kept and logs bad blocks so they cannot be used again in future writes. Pool reserve – Where if a write fails to a block it can be re-routed to the pool of reserved blocks and written there. Track usage  – Blocks on the media can be tracked in a least recently used queue of some sort. The data structures for the queue itself must be wear leveled as well as this queue information is constantly changing. Source:  Wikipedia and Nexcopy Inc. duplicator manufacturer. Continue Reading

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