If you’ve been looking at how much storage space to keep unused on an SSD, you’ve probably come across recommendations that range from 0 to 50 percent and may be frustrated with figuring out the right amount. If you have a 1TB or 2TB drive, it may seem wasteful and costly to leave even 20 percent (200GB or 400GB) unused.
For most consumer use cases and even many professional ones, you can err on the low end of empty storage, even filling a drive to what you thought was 100 percent full, depending on how you currently use and plan to use the SSD. to in the future. After reading this column, you can decide not to worry about free space or choose to keep nearly 30 percent of your SSD empty.
Let’s start with the gory details. (If you want to avoid these and skip advice, skip to the section after the one below.)
Why an SSD needs empty space
SSDs are quiet, energy efficient, long lasting and resilient, but they will eventually fail, just like a hard drive (HDD), although in a very different way. This 2019 analysis from storage and backup company Backblaze remains an in-depth, not-too-technical read on the differences between the two types of storage.
An HDD has many internal moving and rotating parts, while an SSD is ‘solid state’ and everything happens as a result of an electrical action in the chips of the drive. More specifically, reading data from an SSD memory cell uses a very low voltage and causes no real wear and tear; writing data requires higher voltages that eventually wear out the storage bits.
SSDs have an estimated finite number of times each cell can be written, along with a generally expected total number of writes to the drive over its lifetime. Because writing data involves magnetic changes, an HDD does not experience the same level of wear when updating files in an identical location on a drive. (You can use a utility like DriveDx to do a running estimate of the remaining life of an SSD or HDD. It has some Apple-related limitations for monitoring external drives.)
SSD firmware rotates through memory cells, units that store 1 to 4 bits each, and evens out wear across the drive. Otherwise, a frequently used cell would burn out well ahead of other cells. Whenever you save a document, copy files, or otherwise cause data to be written to an SSD, or when the operating system takes an automated action of the same sort, the cells the SSD writes to are completely different from where the previous data was. was saved. The SSD firmware keeps track of all this – it’s seamless for the OS and you.
Complicating this is that SSDs group memory cells into larger units known as: Pagesand pages are grouped in blocks† Depending on the design of the SSD chips, a page can contain 2K to 16K and a block can be between 256K and 4MB. Because of the way free storage is distributed, when an SSD writes data, it may only write the value of a page, or it may need to write an entire block. †
Overprovisioning: room to extend the life of an SSD
That overhead combined with a number of cells that failed early led manufacturers to overcommission the storage by building in extra capacity that you (and the operating system) never see. This invisible part allows an SSD to write smaller pages more often than larger blocks, preserving the overall longevity. An article by disk maker Seagate compares it to the game of 15 squares.
This overcrowded storage is hidden in disk marketing by exploiting the difference between the powers of two and the powers of 10. A 500 GB drive provides 500 billion bytes of storage space. However, memory chips are expressed in powers of 2. The closest value to a billion is a “gibibyte” or GiB, based on units of 1024 (2^10): 1 GiB is 1,073,741,824 bytes. A 500 GiB drive contains 537 GB of storage, but you only see 500 GB – that extra 37 GB is the inherent overprovisioned amount for the disk.
For most consumer purposes, even when an SSD is your boot volume, you could use 100 percent of an SSD’s storage, as shown in the Finder, and still experience a long and happy life from your drive.
SSDs have an estimated lifespan of about 5 to 10 years in normal use based on a number of written terabytes (TBW), which is how a drive should function with well-distributed writes over a given amount of data. Featuring Samsung’s affordable T7 series of external drives, the 1TB model is rated at 360TBW, which equates to an average of 200GB of data written every day for 5 years. Higher capacities have higher TBW numbers, as they are expected to undergo more writes relative to their size.
Samsung also offers a short and understandable white paper on overprovisioning aimed at data center users, but with an incredibly useful three-line diagram that will help you decipher the usefulness of more unused storage space on a disk. These factors are critical in data centers where SSDs undergo much greater amounts of writes than on a personal computer.
Without overprovisioning, something only available in data center oriented drives, with no secret SSD stash, the longevity factor is shown as 1.
Shift to 6.7 percent, the amount Samsung and other SSD makers bake into their consumer drives, and the longevity factor more than doubles to 2.09. That’s the baseline used for Samsung’s TBW figures for its consumer-grade SSDs.
Save a total of 28 percent and the factor jumps to 5.22. That can take a ride of 5 years and extend it to about 12 years. But you’re more likely to want that overhead on an SSD that you use much more intensively for writing files than an average user, such as one used for daily live recording or audio and video editing.
How much storage space should you set aside as empty? Let’s take a look at that.
How much to overprovision?
Here’s a quick rundown of recommendations for overprovisioning, excluding the roughly 7 percent built into Apple, Samsung, and other consumer SSDs:
5 to 20 percent for a Mac startup volume or external drive, depending on usage intensity
A higher percentage could be justified for M1-series Macs
Nearly 0 percent for external drives usually used for offloading data that is read later but rarely rewritten
A boot volume on a Mac will take a lot more writes than most external drives. You can choose to always keep a significant portion of your boot volume empty, from 5 to 20 percent on top of the inherent amount built into the drive. I’d settle for the lower range for normal use and the higher if you’re using software that constantly writes and moves files to disk.
It’s always a good idea to keep some storage space available for a macOS boot volume anyway. Apple will fill empty space on the main volume partition as needed for temporary files, “swap” files (used when memory is under pressure to write data to disk), and Time Machine snapshots before transferring them to a Time Machine volume .
Unused storage on an SSD is treated as free space: For writing data, storage not allocated by the operating system counts towards overprovisioning.
There’s an extra bit of concern to keep the built-in SSD on the M1-series Mac happy for as long as possible. Apple designed the M1-series chips so that a Mac cannot boot if the internal SSD fails because it stores the provisioning information needed to authorize booting from an external drive. If your internal SSD fails prematurely, you can’t just switch to an external SSD like with an Intel Mac. Take that into account in your use.
On an external SSD that isn’t being used to boot your Mac, you should also make your decision based on usage intensity. If you transfer data to an SSD or use it as a Time Machine volume, which adds data incrementally and only overwrites it when necessary, you can achieve full utilization without worrying about shortening its lifespan. With a drive primarily used for reading data, it will experience very little wear and tear.
However, if your external drive is constantly flooded with reads and writes, especially for large files that have been erased, modified, or moved, you’ll be left with a margin, even a significant one, unless you’d rather use all the space and budget for a faster replacement cycle.
This Mac 911 article answers a question from Macworld reader Donald.
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