DWPD (Drive Writes Per Day) Explained
SSD drives are all made using NAND flash and they are priced according to the number of DWPD Drive Writes Per Day. So, a consumer SSD would have far lower DWPD than an enterprise drive. DWPD is based on the number of cells used to hold a single bit of data.
Most of today’s storage arrays use Non-volatile NAND flash memory which is supplied as SSD drives. There are now four types of NAND and these are single-level cell (SLC), multi-level cell (MLC), triple-level cell (TLC) and quad-level cell (TLC) technology.
- SLC stores one-bit-per cell has longer endurance but is significantly costlier to produce with higher capacities. Enterprise Class – 25 DWPD
- MLC uses two bits per cell, the most common type of SSD used by the flash storage vendors – Enterprise Class – 10 DWPD
- TLC uses three bits per cell. These flash technologies have lower endurance, but hold larger capacities and can be produced at lower costs. Consumer Class – 3 DWPD
- QLC uses four bits per cell. These flash technologies have the lowest endurance, highest capacities and lowest cost. Consumer Class – 1 DWPD
We require 10TB’s of flash storage.
1TB MLC-10-DWPD with 5-year warranty – £500 per drive = £10,000
500GB TLC-3-DWPD with 3-year warranty – £150 per drive = £3,000
10 x 1TB x 10 x 5 x 365 = 182.5 PB’s can be written to the flash
20 x 500GB x 3 x 3 x 365 = 32.85 PB’s can be written to the flash
As you can see from the example both provide the ability to write a considerable amount of data during the lifetime of the SSD and effectively you could buy 3x more flash for the same money using TLC on the understanding that after 3 years it will be worn out.
What we don’t show is the drive performance which is always faster using fewer cells.
MLC – So for every write operation we need to perform 2 erases passes and 2 writes one for each bit
TLC – So for every write operation we need to perform 3 erases passes and 3 writes one for each bit
Flash stores data by using an electrical current to etch into Silicon a data bit and this causes Wear Levelling, whereby after so many program erase/write cycles the Flash wears out and this could be 10,000, 100,00 or 1,000,000 writes depending on the type of Flash Storage used. Manufacturers overcome this problem in many ways by using sophisticated algorithms to work out how many times each cell has been used and then automatically re-map those blocks to another portion of Flash Storage or by over-provisioning, this sets aside extra physical flash capacity for background operations, resulting in better write performance and higher endurance.
There are three types of Wear Levelling:
- No Wear Levelling – Nothing is done with the cells and it wears out
- Dynamic Wear Levelling – Data is written evenly across the whole of the Flash
- Static Wear Levelling – Works out how many times a cell has been written and dynamically moves it.
Higher Capacity SSD
To achieve higher capacity SSD drives the silicon die uses a much smaller process maybe 20nm or even lower. For SSD drives this is not so good, sure we pack far higher cells within a given area but the size of transistors and gates within the silicon are much closer together and will not provide the endurance levels a 30nm SSD with a smaller capacity can endure.
Flash storage arrays can provide far higher performance than a comparably priced disk sub-system. Where flash pricing differs is how you intend to use it. If your application uses a lot of writes then you should consider SLC or MLC storage, if your application is an even split of reads/writes then MLC or TLC can be considered.
In essence, it is all down to workload, if you get this right then you could save a considerable sum of money, whilst the flash storage is delivering the performance you need!
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