Wear

Comparing DWPD to TBW

TBW-DWPDA couple of specifications for SSD endurance are in common use today: Terabytes Written (TBW) and Drive Writes Per Day (DWPD).  Both are different ways to express the same thing.  It seems that one vendor will specify endurance using TBW, while another will specify DWPD.  How do you compare the two?

First, some definitions.  “Terabytes Written” is the total amount of data that can be written into an SSD before it is likely to fail.  “Drive Writes Per Day” tells how many times you can overwrite the entire capacity of the SSD every single day of its usable life without failure during the warranty period.  Since both of these are guaranteed specifications, then your drive is most likely to last a lot longer than the number given by the SSD’s maker.

To convert between the two you must know the disk’s capacity and the warranty period.  If  drive maker gives you TBW but you want to know DWPD you would approach it Continue reading

SSDs that Don’t Wear Out

The End of NAND Flash Wear?This is a bad day for The SSD Guy.  I just finished publishing an eight-part series explaining How Controllers Maximize SSD Life, then my evil twin The Memory Guy today published a post telling of a new flash design from Macronix that might just eliminate the flash wear-out mechanism!

But my concerns are inconsequential compared to the feelings of all those folks who have devoted phenomenal time and energy to develop wear management algorithms.

This all stems from an article in the IEEE Spectrum that details a flash chip design that Continue reading

SMART Optimus Ultra+ SSD: SLC Performance Using MLC Flash

SMART Guardian vs Normal Flash Management - More good BlocksSMART Storage Systems has introduced a new enterprise-class SSD that the company says: “increases the endurance of cMLC Flash to a level that makes SLC drives obsolete.”  That’s a pretty hefty claim!

The new Optimus Ultra+ SSD is specified at 100K read IOPS and 60K write IOPS, through its 6Gb/s SAS interface.  With capacities ranging from 100-800GB, this SSD supports up to 50 full drive writes per day over its 5-year lifespan, double that of the company’s Optimus Ultra which was introduced in February.  That’s quite something for an MLC-based SSD.

SMART has tapped into its Guardian technology to reap SLC benefits from MLC flash through both enhanced external and internal algorithms.  Like all other SSD makers and SSD controller makers SMART has focused a lot of attention on error correction, DSP, and other means of correcting errors externally to the flash.  The company has also partnered with Continue reading

What Happens when SSDs Fail?

What happens at the end of an SSD's life?There’s a lot of “Fear, Uncertainty, and Doubt” – FUD – circulating about SSDs and their penchant for failure.  NAND flash wears out after a set number of erase/write cycles, a specification known as the flash’s endurance.

While some caution is warranted, a good understanding of how SSDs really behave will help to allay a lot of this concern. Continue reading

SSDs and RAID

RAID ConfigurationThe SSD Guy has been asked a number of questions lately about SSDs and RAID.  Most of these center around the difference in failure behaviors between SSDs and HDDs – HDDs fail randomly (if ever), while SSDs fail relatively predictably due to wear.

Oddly enough, SSD failures due to wear make them a little friendlier than HDDs.  The wear mechanism is managed by the controller in the SSD.  SSDs have spare blocks, and the controller manages those blocks, so the controller understands exactly how much wear the SSD has undergone and how much room is left before the SSD will start to have difficulties. Continue reading