SSD endurance and warranties

TBW is the total volume of writing a maker warrants a drive to absorb before its endurance cover ends. For ordinary desktop work that allowance is far larger than what you spend, and the machine is usually replaced first. Endurance is real. It rarely decides a purchase.

What TBW and DWPD are counting

TBW — terabytes written — is a lifetime total. It counts what the host sends to the drive, not the extra writing the drive does on its own behalf as it shuffles data around the flash. That amplification sits on the maker's side of the line: the rating is stated in host writes, under a standard client workload, so you can hold it against what your machine actually sends.

DWPD — drive writes per day — is the same promise divided up. It is how many times you could fill the entire drive, every day, for the length of the warranty, and still be inside the rating. Because it is defined over a capacity and a period, it says nothing on its own: one drive write per day on a small drive warranted three years is a smaller allowance than a third of one on a large drive warranted five.

The two convert exactly. Multiply DWPD by the capacity in terabytes, by 365, and by the warranty years, and you have TBW. Divide TBW by those same three and you have DWPD. Consumer datasheets tend to print the total, enterprise ones the daily rate, and putting a pair of drives on one footing is arithmetic rather than judgement.

What neither figure describes is the flash underneath it. TLC, QLC, and DRAM-less SSDs works through why the printed rating belongs to the whole drive rather than to its cells, and why comparing two ratings is not the same as comparing two cell types.

How much a normal machine writes in a year

There is no useful average to quote here, so measure instead. Nearly every drive keeps a running count of how much the host has written to it, among the rest of its self-monitoring data — a USB-bridged drive is the common exception. Read that total, set it against how long you have owned the drive, and you have your own rate. Divide the rating by it and the answer arrives in years.

Use calendar time you have owned the drive as the denominator, not the powered-on hours the drive also reports. That counts time it was awake rather than time it was yours, so on a machine that spends most of the day switched off it makes your daily rate look much higher than it is.

The other direction makes the relationship easier to feel. Divide the rating by the capacity and by the days the warranty runs, and you get how much of the drive you would have to fill and empty each day to spend it on schedule. On a consumer drive that is a sizeable fraction of the whole drive, daily, for years. Ordinary desktop work does not go near it. Your writing arrives in bursts — an operating system upgrade, a few large downloads, a backup — with long stretches of reading in between.

Drives also publish their own reckoning of how much life they have used, as a percentage. Take it as an estimate, because that is what it is: the firmware judging its own wear from the cycles it has run and the errors it is correcting, by a rule the maker chose. It is the best figure you have. It is not a measurement of the flash that is left.

What the warranty actually covers

A drive's warranty runs for a stated number of years or up to the endurance rating, whichever comes first. Reaching either one ends it. That is the half buyers miss: a five-year drive is not covered for five years if you write the rating out of it in two.

Which limit you would reach first tells you which figure to shop on. If your writing will not spend the rating inside the term, the endurance number is doing nothing for you and the years are the whole of the cover. On a desktop that is the usual case.

Terms differ between makers, so read the ones attached to the drive you are buying rather than the ones you remember. Typically the warranty covers the drive and not what was on it, which is why a warranty of any length is not a backup. Typically the remedy is a repaired or replacement drive rather than money back, and typically the cover is tied to the original purchase, so it does not follow a drive sold on.

When endurance is worth paying extra for

The workloads that spend a rating write continuously rather than occasionally: video recorded or transcoded onto the drive, scratch space for an editing timeline, a database or anything else logging steadily, a machine hosting virtual machines. The writing is the work rather than a side effect of it.

Gaming is not one of them, however large installs have got. A game is written once and read thousands of times, and reading does not spend the write budget. The same holds for a media library or a photo archive: filled in a few long runs, then read from for years.

If you are in the writing-heavy case, more capacity is often the cheaper way to buy endurance than a model sold on its rating. The rating scales with the size of the drive. Sizing an SSD covers what else that room buys you.

Which of the two is the better buy is then a price question rather than an endurance one. When an SSD deal is actually a deal covers how to put two drives of different sizes on one footing before you decide the extra is worth paying.

Why the figure is missing from most listings

Endurance is not something retail listings carry. Makers publish it on a datasheet, stated separately for each capacity, and it seldom survives the trip to a product page. Warranty length travels a little better, though often as a line of marketing copy rather than as a field.

So where a listing states neither, we leave the field empty rather than filling it in from a datasheet for a drive of that name. An empty field records what the listing said. It is not a claim that the drive has no rating.

If you want the number, the maker's datasheet has it — check the figure you find is quoted for the capacity you are buying, not the largest in the range.

How to choose an SSD puts endurance in its place among the four questions that decide most purchases, and every SSD we track is on one page.