Page 3 : SSD Endurance & Maintenance
The 1TB WD Blue SSD is rated for a lifetime write lifespan of 400 TBW (Terabytes Written). That does not seem like a lot (equal to overwriting the drive just 400 times), but it is considered enterprise-grade endurance. Many consumer-grade SSDs of equivalent capacity are rated at around 75-100 TBW.
Based on a typical consumer DWPD (Drive Writes Per Day) of 20 GB per day, this 1TB WD Blue will last at least 54 years. Please note that this long lifespan is due to the large capacity. The 500 GB and 250 GB WD Blue SSDs are rated at 200 TBW and 100 TBW respectively. That corresponds to an estimated lifespan of 27 years and 13.5 years respectively.
Like all other current SSDs, the 1TB WD Blue SSD comes with certain features to help extend its lifespan :
Unlike hard disk drives, flash-based SSDs write and overwrite data in large blocks of 512 KB to 1 MB in size. Even if you only need to write one byte of data, it has to erase and overwrite an entire block. This causes a lot of wear on the memory cells and greatly reduces their lifespan.
To help extend the lifespan of the drive, SSDs perform wear levelling by spreading the writes, so that the flash memory cells have equal wear. The lifespan of the memory cells remain unchanged, but it prevents some of them from failing earlier due to excessive wear.
Write Combine Cache
SSDs also use a write buffer to temporarily store and combine the writes before they are actually written to the flash memory. This reduces the number of block erases required, and consequently, extends the lifespan of the flash memory cells.
The 1TB WD Blue SSD boasts a large 1 GB DDR3L memory cache, which should help to really extend the limited lifespan of the TLC flash memory. This allows the 1TB WD Blue to get away with a very small amount of “over-provisioned” space.
Current SSDs support the TRIM command, otherwise known as the ATA8-ACS-2 DATA SET MANAGEMENT command. Operating systems that support TRIM (e.g. Microsoft Windows 7) will notify the SSD when data blocks are deleted in the file system. This allows the SSD to perform garbage collection in the background – internally erasing the affected blocks so that they are ready to be written to.
Without the TRIM command, the SSD will not know when a block of data has been deleted by the operating system. When new data is written to the same block of data, it will force the SSD to perform the time-consuming read-erase-modify-write cycle, which not only cripples performance but also increases wear on the affected memory cells.
This is a new SSD technology that was introduced in May 2015, as part of the T10 SCSI Standard. Multi-stream greatly improves performance and extends lifespan by reducing or even eliminating garbage collection.
It achieves this by marking data writes that are associated with one another, or have a similar lifetime, with a unique stream ID. This allows the SSD controller to pack all data writes with the same stream ID into the same block.
When the operating system deletes data, it is likely that they are all packed into the same block. If the block has not been written to the SSD, then this eliminates the pending write operation completely. If the block has been written to the SSD, then this would only require that single block to be erased, instead of multiple blocks (which would happen if the data was not all packed into the same block).
First of all, you should never, ever defragment solid state drives. Spatial fragmentation of data on the SSD has no effect on its performance. Fragmented data are accessed as quickly as nicely-packed blocks, so it’s pointless to defragment the data blocks. Doing so will only reduce the lifespan of the flash memory cells by putting them under additional wear.
Remember – TLC flash memory will only last about 1,000 erase/write cycles. You will want to minimise the number of times each flash memory cell is erased.
You should also use an operating system that supports the TRIM command. If you are using one of the following operating systems, then you have nothing to worry about :
- Microsoft Windows 7, or better
- Microsoft Windows Server 2008 R2, or better
- Linux 2.6.33, or better
- FreeBSD 8.2, or better
- Mac OS X Snow Leopard, or better
If not, you should consider upgrading your operating system. Otherwise, you will need to perform manual garbage collection on a regular basis, either using a manufacturer utility, or newer defragmentation software that specifically supports solid state drives. Basically, these utilities will retrieve the list of free blocks from the operating system’s file system and pass it to the SSD in the form of TRIM commands, so that it will know which blocks to erase internally.
Western Digital does not provide such a utility for their WD Blue SSDs, so if you are using an older operating system, you will need to use a third-party SSD optimisation software. Note that those software are not necessary if you are using an operating system that supports TRIM.