SSD ENDURANCE

samsung_pm1643_ssdThere has been a lot of development in the SSD market over the years. Today modern SSD products are very inexpensive but people should not take them for granted with their large MTBF ratings.

The first SSD products suffered from firmware problems which adversely affected performance etc.  Some years have passed and new chips and logic have immensely improved the technology. SSD drives made since 2009 are generally problem free.

The industry generally now has a reserve of some 5-8% of storage blocks held in reserve that are not available to the system. The SSD controller uses this reserve to replace bad blocks. This not unlike the way hard disks use spare sectors to replace bad ones. The goal is to prevent bad blocks from adversely affecting the operating system which potentially could crash due to a read error.

Consumer focused SSD drives today use 32-layer TCL NAND cells to make the chips comparatively low cost. More recent 48-layer devices are still comparatively expensive but average selling prices are falling rapidly.

The trend seems to be for more layers at the expense of endurance. Work on a QLC with 4 bits is being developed but endurance is still relatively poor. The larger density requirements has attracted a lot of research.

Each flash memory cell can only be written so many times before it becomes unreliable.  A general rule of thumb is that leaving 20% of the SSD empty will allow the device abundant room for wear levelling. The Windows swap file probably is the worst for stress on the cells so having lot of free space allows the SSD controller some breathing space.

Having less free space will shorten the effective life of a SSD very quickly. With only 10% free the typical TLC SSD may last 18 months before the cells are worn out. With 20% the SSD can last over 5 years and with 30% free it can last over 10 years. The wear depends on the the amount of writes . A typical laptop are rather low with the Windows swap file being the main source of writes. More RAM reduces the swap file activity considerably.

The controller has a lot of work to do as it has to manage writes. For the most part the work is not very demanding as the table of blocks is checked to see how much use each has endured. The SSD has to constantly move data around to maximize overall endurance.

SSD endurance is commonly rated in TBW which is simply the number of TB written. Most consumer SSD drive have 100 or more TBW endurance ratings. Using a program like Crystal Disk Info can show the GB read and written.

For example, if your drive is rated for 365 TBW, that means you can write 365 TB into it before you may need to replace it. If its warranty period is 5 years, that works out to 365 TB ÷ (5 years × 365 days/year) = 200 GB of writes per day. If your drive was 200 GB in size, that’s equivalent to 1 DWPD. Correspondingly, if your drive was rated for 3.65 PBW = 3,650 TBW, that works out to 2 TB of writes per day, or 10 DWPD.

New model SSD drives are coming out in droves. One area that has caught everyone’s attention is the improvements in endurance, We suspect overprovisioning will be the most common way to extend service life but we again mention that backups are important.

SUPERFETCH

Superfetch was first introduced with Windows Vista. Open up services and disable superfetch. This will reduce the number of writes to a SSD.

Samsung, Kingston and others also have tools for their respective products. These are helpful to spot problems etc.

Remember SSD drives also can show bad blocks so its important to have backups of important data.

TRIM

Windows 10 has good support for SSD and the drive optimizer can run the TRIM command on a SSD weekly  schedule to endure the SSD and Windows are working together to maximize the endurance.

BIGGER DRIVES LAST LONGER

The larger capacity model SSD drives last proportionally longer. An affordable 120GB SSD will still last a long time in a laptop, but the larger 240GB SSD will last longer. The larger 480GB SSD will last even longer.

Realistically the prices of larger capacity SSD drives fall faster than their wear out from use.  With lower prices many have purchased larger capacity SSD drives. The upgraded SSD extends durability.