It’s been a couple of months now since reports began to appear of M1 Mac users experiencing excessive SSD usage. In the worst cases, users had already reached 10% of the rated endurance capacity of their drive within 2 months.
— William Li (@_wli) February 25, 2021
Why is having large amounts of data written to the SSD bad?
An SSD stores data in NAND cells. Over time, these cells experience wear as data is written or erased.
Once they reach a certain level of wear, they can no longer reliably store data. The more data that’s written, the faster they’ll wear out. Manufacturers usually state the rated endurance of SSD’s in TBW, or TeraBytes Written, which is the total amount that can be written to a drive over its lifetime.
How much data are the M1 Macs writing?
We ran some tests, comparing a late 2020 M1 MacBook Air and a 2016 Intel MacBook, both with 8GB of RAM, running several applications. We found that, on average, across all applications, the M1 MacBook Air wrote 20 times more data than the Intel Mac with the same amount of RAM under the same workload. In the worst case, editing three photos in Adobe Lightroom Classic (running under Rosetta 2 on the M1 Mac) saw the M1 Mac writing 58 times more data to the SSD than the Intel Mac. In other applications, running natively on both processors, the difference was between 8 times and 32 times.
How will this affect the life of the SSD in the M1 Macs?
Assuming the same SSD endurance, writing 20 times more data to the SSD will inevitably mean that it reaches its rated endurance threshold 20 times quicker. Once that threshold is passed, the likelihood of SSD failure increases exponentially, and the possibility of errors and data loss is much greater.
Why is this a bigger problem than it appears?
Although previous models of Mac released since around 2015 have also had soldered, non-replaceable storage, they were still able to be booted from an external SSD once the internal drive failed.
With the M1 Macs, this has all changed. The internal SSD on an M1 Mac contains several vital containers without which the computer won’t boot. These containers can only exist on the internal drive. Once the drive fails, the Mac will no longer boot, not even from an external drive.
The first reported dead M1 Mac SSD
In March, the first report of an M1 Mac to have died from this issue appeared. It was an M1 MacBook Pro with a 512GB SSD and 16GB of RAM. It had seen 512GB written to the SSD over the 4 months of its life.
Here's smartctl. Rounded most of the numbers slightly as it's not my machine. Was used for development and local testing I believe. Not too many apps installed, they do use a heavier IDE and a file sync app, but generally seems like plenty of free RAM. pic.twitter.com/BkrPkiFV1h
— Ryan Hileman (@lunixbochs) March 1, 2021
Estimating the endurance level of Apple’s SSD
Apple don’t publish endurance ratings for their SSD’s. However, we can estimate the endurance in a number of ways.
First, we can look at how long the SSD’s that have already failed lasted. The dead Mac reported above has a 512GB SSD and had seen 511.9GB written before it died. This implies an endurance rating of 512TB for the 512GB drive. Assuming the same NAND chips, proportionally, the endurance rating of the 256GB drive in my M1 Mac would be 256TBW.
Another way to estimate the endurance is by looking at the type of NAND used, and at the ratings for other SSD’s using the same or similar NAND.
The type of NAND used in the M1 Macs can be checked in IORegistryExplorer (part of Hardware IO Tools for XCode).
In my case, the NAND used in my M1 MacBook Air are 3D TLC NAND, manufactured by SK Hynix. Hynix’s Gold P31 SSD, released late last year, uses their latest 128-Layer 3D TLC NAND, and is possibly the the same, or similar, NAND to that found in my M1 MacBook. That drive has an endurance rating of 500TB for the 500GB version. This, again, would put the endurance rating for the 256GB drive in my M1 MacBook Air at 256TBW.
My M1 MacBook Air is writing on average around 700Gb per day to the SSD. So, if this 256TBW endurance is accurate, then I could expect my M1 MacBook to pass that threshold after exactly 1 year at current usage – that’s 256TBW divided by 0.7 Terabytes per day = 365 days.
It’s still common to see 2012 and even earlier MacBooks still in use today. Often they’ve been upgraded with a new SSD and extra RAM. Sadly, unless this SSD write issue is addressed soon, it’s unlikely that we will see many M1 Macs still in working condition, let alone still in use, 9 years from now.