Apple introduced its Fusion Drives in the Late 2012 models of iMac, and has offered SSD (solid-state drive) storage as an option or standard in MacBook Air models since January 2008, and the MacBook Pro since late 2008. There are a lot of Macs out there using SSDs, and some of them are now getting pretty old.

So, a simple question, how long can you expect an Apple-fitted SSD to last?

The answers that I can find range from 1-3 years, up to more than 100.

Most SSDs – including those fitted by Apple, as far as I can tell – use non-volatile flash memory units, chips, instead of the spinning platters that make up a regular hard drive. With no moving parts, SSDs are not of course prone to the mechanical failures which so often end the life of hard drives. Because they store data in electronic circuits, they do not suffer from physical problems in the magnetic coating on a hard drive’s platters.

Instead, two factors limit the lifetime of an SSD: flash memory can only be written to a certain number of times, and ultimately all flash memory fails to retain its data after about 10 years anyway.

There are several mechanisms used to optimise performance and lifetime of SSDs, the most important of which are TRIM and wear levelling. TRIM (which is built into OS X) is designed to discard blocks of memory no longer being used for storage so that they can be re-used, and wear levelling ensures that each block of memory receives a roughly equal number of write operations, to spread its use (and ageing) evenly.

In Fusion Drives and SSDs used as the startup disk, all the active files in OS X will be stored on the SSD, to ensure best performance. Some files within OS X are written to very frequently. If they were allowed to reside in the same blocks of memory all the time, then those blocks would quickly run out of available writes, and would fail long before other blocks of memory.

Wear levelling moves the files around different blocks of memory in the SSD, to even out its ‘wear’. Inevitably, moving the busiest files around involves more write operations to the SSD, so there is a delicate balance between over-zealous levelling, which would shorten overall life, and none at all.

Let’s say that your SSD is good for 10,000 writes to each block of memory before it stops writing. If there were no wear levelling at all, and OS X wrote to one particular block 100 times each day, then that block would only last 100 days before it stopped working, and the SSD failed.

If wear levelling did not incur any write overhead, and was perfect, then you would have to write the total capacity of the SSD out 10,000 times before it stopped writing: for a standard Apple-fitted 120 GB SSD (e.g. in a Fusion Drive), that would be 1200 TB of writes. Even if, in practice, wear levelling is only 50% efficient, that SSD should be good for 600 TB of data to be written to it.

The Fusion Drive in my iMac has been run for a total of 4,100 hours, which is almost 6 months. Over that time, 6 TB has been written to the 120 GB SSD in that Fusion Drive. So on current write usage, that SSD should last for around 100 of those 6 month periods before it can no longer write. That is a lifetime of 50 years. Even if I used the SSD much harder, writing around 50 TB/year to it instead of 12 TB/year, it should last well over 10 years before the SSD is likely to fail. Because all flash memory is likely to fail after about 10 years, it is not likely to die because it has been written to too many times.

This depends on reasonably efficient wear levelling. We do not know how good the wear levelling has been in Apple SSDs and Fusion Drives over their history.

One indicator of this is the wear levelling count, which is reported on all SSDs by good S.M.A.R.T. reporting software such as DriveDx. It is not reported by Disk Utility, nor in System Information.

I have just seen a DriveDx report on one SSD from an Apple Fusion Drive which has been running for a total of just over 2,000 hours – that’s around 3 months – and in that period has written almost 500 TB to that 120 GB SSD, which I find extraordinary. Consequently its wear levelling count is now below 50%, and according to the S.M.A.R.T. health indicators, the SSD is in danger of failure.

Going by those figures, the average amount of data written to that SSD over its running lifetime has been 250 GB per hour, or 4.2 GB per minute, or nearly 70 MB per second, which seems unbelievable even for a heavily-used database server. Such a short working lifetime of only 3 months of continuous use would suggest a serious failure, probably in wear levelling.

If you have an SSD as your startup drive, or a Fusion Drive, you might like to make a rough estimate of the projected lifetime of your SSD: using DriveDx or a similar tool, take the size of your SSD in GB and multiply it by 5: that gives you the approximate quantity of data in TB which can be written to it before it is likely to start to fail. Compare that with the actual total data written in TB, given in S.M.A.R.T. indicator number 175, Host Write MiB.

The wear levelling count given in S.M.A.R.T. indicator 173 normally falls to around 90-95% in early use, but should then decline only very slowly. If it is approaching 50%, or even below, your SSD is wearing out.

Having come across one older iMac with a Fusion Drive which seems to be wearing out prematurely, I wonder if others may be similarly affected. If there was a bug in older versions of the wear levelling software, that could well be the case.