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'Wear levelling' - a bedroom aid for multi-layer cell Flash

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Wear-levelling

Wear-levelling algorithms are used to reduce the likelihood of particular blocks being used up, having their maximum number of writes reached, and so falling out of service and reducing the capacity of the SSD, and its ability to free up blocks for fresh writes.

With dynamic wear-levelling written data is put in blocks from the free block pool. Garbage collection patrols the existing blocks and allocates ones with deleted (invalid) data to the background erase processing after which they are added to the free block pool.

However blocks containing static, unchanging data just sit there and don't get rewritten. We could imagine that over a period of, say 6 months, such blocks receive zero writes whilst other blocks could receive, let's be dramatic, 200 writes, this creating an imbalance. Static wear-levelling locates these static data blocks and moves the data to the more often written blocks, transferring their data to the previously static data blocks.

Then, over the next six months the previously static blocks get 200 writes and the previously well-used blocks get no writes. At the end of a year both sets of blocks have had 200 writes; a levelled wear number between the two sets of blocks.

All the foregoing applies both to SLC and MLC NAND. The problem with MLC NAND is that its endurance is less than SLC NAND, For example, Samsung has suggested SLC flash can support up to 100,000 writes, 2-bit MLC is a tenth of that at 10,000 writes and 3-bit MLC is ten per cent of that at 1,000 writes. Extending this trend would have 4-bit MLC supporting 100 writes; clearly a complete no-no for its deployment unless radical measures are taken.

SandForce says its controllers level the amount of writes across flash blocks. The controllers have a recycler for garbage collection, and its says its DuraWrite technology optimises the number of program cycles and this can, SandForce claims, extend the endurance of its flash by up to 20 times compared to other controllers.

Over-provisioning

The main point of basic wear-levelling is to ensure an equality of write numbers across the blocks in the SSD. Above and beyond that there are other techniques used to extend endurance.

One technique is to over-provision the flash, an opposite of the thin-provisioning idea seen in shared storage arrays. An SSD with a nominal capacity of 200GB may actually have 250GB capacity, with the extra 50GB hidden from the host system and used solely at the discretion of the SSD controller. As flash blocks in the SSD wear out they are mapped out of use by the controller, and a new block added to the general free block pool from the 50GB reserve.

There is a limit to how long this will work because, eventually the 50GB reserve is used up and the SSD then faces a slow death as blocks fail one after the other. If the SSD is targeted at a known application, such as a consumer media player then its makers know most writes will be of long sequential files, music tracks or videos, and they can predict how long a given amount of flash will last if they assume an average number of bytes of data is written per day.

With a combination of wear-levelling and over-provisioning they can produce flash for a consumer device that could last say, five years with 500GB of data being written per day.

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