Understanding SSD: Solid State Drives

The general idea: SSDs are faster and more efficient than Hard Disk Drives but also more expensive. We want to use SSDs whenever we can on machines that have lots of I/O (like web servers or databases).

The Basic Differences between SSDs and HDDs

+ SSDs do not have moving parts and no rotating platter. They are entirely flash memory based which is non-volatile (meaning they retain information with the power off)

+ They rely on an embedded processor/controller to perform read/writes instead of an actuator arm on HDDs.

+ A good SSD can perform read/writes around 550MB/s HDDs usually run around double digits.

+ Uses less power, less heat, and doesn't get affected by magnets.

+ SSDs overprovision memory space for reserves to replaces memory cells that no longer work (A 512 GB drive will have 32 GBs reserved so in actuality, only 380 GB is usable).

Understanding SSD Endurance

Though more efficient, SSDs still have a mortal time on this Earth just like HDDs. In the SSDs case, it is due to the flash memory cells becoming unable to store data. Usually when researching SSDs, try to look for the stat TBW or DWPD to find the endurance of an SSD. You can usually find the TBW (Terabytes written) stat on SSD datasheets from company websites. TBW tells the consumers how many terabytes can be written to the disk before it will begin to malfunction. DWPD (Drive writes per day) tells us how much of the disk can be written to everyday and still be expected to function within the warranty period. The equation is:

DWPD = (TBW of an SSD * 1000)/(365 days * years of warranty * capacity of ssd in GB)

Endurance Example

Say we had the Crucial MX300 SSD with a three year warranty. The datasheet found on crucial tells us that the TBW for their 1TB SSD is 360 TBW.

That means 360 terabytes can be written to the disk before it will begin to malfunction.

In terms of DWPD, the SSD can be calculated using the DWPD formula above:

DWPD = (360 TB * 1000 Gb/TB) / (365 days * 3 years * 1000 GB)
DWPD = 360,000 / 1,095,000
DWPD = 0.3288 (drive writes per day)

This means that you can overwrite 32.9% of the drive every day for the three year warranty period, which is equal to 329 GBs of the 1TB drive.