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ThinkSystem PM1645 Mainstream SAS 12Gb SSDs
The ThinkSystem PM1645 Mainstream SAS 12Gb solid-state drives (SSDs) in capacities of up to 3.2 TBare next-generation high-performance SSDs suitable for a wide range of applications of running onThinkSystem servers. The PM1645 Mainstream SAS 12Gb SSD is shown in the following figure.

Entry SSDs and Mainstream SSDs have similar read and write IOPS performance, but the key differencebetween them is their endurance (or lifetime) (that is, how long they can perform write operations becauseSSDs have a finite number of program/erase (P/E) cycles). Mainstream SSDs have better endurance butlower cost/IOPS ratio compared to Entry SSDs. SSD write endurance is typically measured by the numberof program/erase (P/E) cycles that the drive incurs over its lifetime, listed as the total bytes of written data(TBW) in the device specification.

The TBW value assigned to a solid-state device is the total bytes of written data (based on the number ofP/E cycles) that a drive can be guaranteed to complete (% of remaining P/E cycles = % of remainingTBW). Reaching this limit does not cause the drive to immediately fail. It simply denotes the maximumnumber of writes that can be guaranteed. A solid-state device will not fail upon reaching the specifiedTBW. At some point based on manufacturing variance margin, after surpassing the TBW value, the drivewill reach the end-of-life point, at which the drive will go into a read-only mode.

For example, the 1.6TB PM1645 SSD has an endurance of 8,760 TB of total bytes written (TBW). Thismeans that for full operation over five years, write workload must be limited to no more than 4,800 GB ofwrites per day, which is equivalent to 3.0 full drive writes per day (DWPD). For the device to last threeyears, the drive write workload must be limited to no more than 8,000 GB of writes per day, which isequivalent to 5.0 full drive writes per day.