Flash Degradation: Fact or myth?
The cost of flash storage is coming down, and with the stellar benefits possible from flash’s increased speeds, many enterprises are contemplating the costs and benefits to their company. But one issue of concern that we keep hearing about that influences the purchase an all-flash storage array, is flash degradation, i.e. how long is the lifespan of a flash memory cell?
Is it a fact or a myth – that flash storage degrades? While it is true that in the early days, flash memory cells were being worn out unusually fast, that was because they were being treated in the same way as a spinning disk is treated when writing I/Os to a disk platter. Writing to the same memory cells over and over again slowly causes the silicon elements in the cell to fail from overuse. This was a very real problem five or six years ago, but the manufacturers have redesigned their controllers to address this.
How are they doing it? A typical flash memory cell contains a memory page that is 4k in size. So, the first mechanism used to help alleviate this issue is to set the erase function to only reset cells in groups of 64 pages (256k). This forces the controller to perform write I/Os that ‘overwrite’ a piece of data to other cells in the module, then mark the ‘overwritten’ page as old. When the array does its background housekeeping processes, it will optimise the location of data within the flash modules so that old pages are cleared and your data is placed where it is best utilised by the array (this is known as ‘wear leveling’). This reduces the risk of constantly writing to the same memory page and wearing out the circuitry.
It should be noted that there is no need to perform any ‘disk defragmentation’ tasks (or running a ‘defrag’) on Flash modules. The storage array controllers do a much better job of this today through their housekeeping tasks than any manual process can through system software. This is now a standard feature on all flash storage arrays.
Most flash storage vendors are now reporting that the mean time between failures (MTBF) rates of flash modules are now equal or better than the equivalent hard drive. So while the phenomenon of flash degradation technically still exists, this is virtually not an issue today because the array controllers are properly utilising their flash storage. As a result, you are now starting to see some vendors extending their standard warranties on flash modules beyond the typical three year period.
Still, it can be hard to wade through all of the conflicting information these days with new technology. It’s important to obtain independent advice and look at the facts objectively: read about how you can make an informed decision when buying flash storage. Discuss your flash storage with an NTT ICT account manager to learn how flash storage meet your company’s needs, and get access to our storage experts for truly independent advice. Contact us online here.
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