Fast SSDs aren’t always fast when writing to them. Even an Apple silicon Mac’s internal SSD can slow alarmingly in the wrong circumstances, as some have recently been keen to demonstrate. This article explains why an expensive SSD normally capable of better than 2.5 GB/s write speed might disappoint, and what you can do to avoid that.

In normal use, there are three potential causes of reduced write speed in an otherwise healthy SSD:

• thermal throttling,
• SLC write cache depletion,
• the need for Trimming and/or housekeeping.

Each of those should only affect write speed, leaving read speed unaffected.

Thermal throttling

Writing data to an SSD generates heat, and writing a lot can cause it to heat up significantly. Internal temperature is monitored by the firmware in the SSD, and when that rises sufficiently, writing to it will be throttled back to a lower speed to stabilise temperature. Some SSDs have proved particularly prone to thermal throttling, among them older versions of the Samsung X5, one of the first full-speed Thunderbolt 3 SSDs.

In testing, thermal throttling can be hard to distinguish from SLC write cache depletion, although thorough tests should reveal its dependence on temperature rather than the mere quantity of data written.

The only solution to thermal throttling is adequate cooling of the SSD. Internal SSDs in Macs with active cooling using fans shouldn’t heat up sufficiently to throttle, provided their air ducts are kept free and they’re used in normal ambient temperatures. Well-designed external enclosures should ensure sufficient cooling using deep fins, although active cooling using small fans remains more controversial.

SLC write cache

To achieve their high storage density, almost all consumer-grade SSDs store multiple bits in each of their memory cells, and most recent products store three in Triple-Level Cell or TLC. Writing all three bits to a single cell takes longer than it would to write them to separate cells, so most TLC SSDs compensate by using caches. Almost all feature a smaller static cache of up to 16 GB, used when writing small amounts of data, and a more substantial dynamic cache borrowed from main storage cells by writing single bits to them as if they were SLC (single-level cell) rather than TLC.

This SLC write cache becomes important when writing large amounts of data to the SSD, as the size of the SLC write cache then determines overall performance. In practice, its size ranges from around 2.5% of total SSD capacity to over 10%. This can’t be measured directly, but can be inferred from measuring speed when writing more data than can be contained in the cache. As it can’t be emptied during full-speed write, once the dynamic cache is full, write speed suddenly falls; for example a Thunderbolt 5 SSD with full-speed write of 5.5 GB/s might fall to 1.4 GB/s when its SLC write cache is full. This is seen in both external and internal SSDs.

To understand the importance of SLC write cache in determining performance, take this real-world example:

• 100 GB is written to a Thunderbolt 5 SSD with an SLC write cache of 50 GB. Although the first half of the 100 GB is written at 5.5 GB/s, the remaining 50 GB is written at 1.4 GB/s because the cache is full. Total time for the whole write is then 44.8 seconds.
• Performing the same to a USB4 SSD with an SLC write cache in excess of 100 GB has a slower maximum rate of 3.7 GB/s, but that’s sustained for the whole 100 GB, which then takes only 27 seconds, 60% of the time of the ‘faster’ SSD.

To predict the effect of SLC write cache size on write performance, you therefore need to know cache size, out-of-cache write speed, and the time required to empty a full cache between writes. I have looked at these on two different SSDs: a recent 2 TB model with a Thunderbolt 5 interface, and a self-assembled USB4 OWC 1M2 enclosure containing a Samsung 990 Pro 2 TB SSD. Other enclosures and SSDs will differ, of course.

The TB5 SSD has a 50 GB SLC write cache, as declared by the vendor and confirmed by testing. With that cache available, write speed is 5.5 GB/s over a TB5 interface, but falls to 1.4 GB/s once the cache is full. It then takes 4 minutes for the cache to be emptied and made available for re-use, allowing write speeds to reach 5.5 GB/s again.

The USB4 SSD has an SLC write cache in excess of 212 GB, as demonstrated by writing a total of 212 GB at its full interface speed of 3.7 GB/s. As the underlying performance of that SSD is claimed to exceed that required to support TB5, putting that SSD in a TB5 enclosure should enable it to comfortably outperform the other SSD.

Two further factors could affect SLC write cache: partitioning and free space.

When you partition a hard disk, that affects the physical layout of data on the disk, a feature sometimes used to ensure that data only uses the outer tracks where reads and writes are fastest. That doesn’t work for SSDs, where the firmware manages storage use, and won’t normally segregate partitions physically. That ensures partitioning into APFS containers doesn’t affect SLC write cache, either in terms of size or performance.

Free space can be extremely important, though. SLC write cache can only use storage that’s not already in use, and if necessary has been erased ready to be re-used. If the SSD only has 100 GB free, then that can’t all be used for cache, so limiting the size that’s available. This is another good reason for the performance of SSDs to suffer when they have little free space available.

Ultimately, to attain high write speeds through SLC write cache, you have to understand the limits of that cache and to work within them. One potential method for effectively doubling the size of that cache might be to use two SSDs in RAID-0, although that opens further questions.

Trim and housekeeping

In principle, Trim appears simple. For example, Wikipedia states: “The TRIM command enables an operating system to notify the SSD of pages which no longer contain valid data. For a file deletion operation, the operating system will mark the file’s sectors as free for new data, then send a TRIM command to the SSD.” A similar explanation is given by vendors like Seagate: “SSD TRIM is a command that optimizes SSDs by informing them which data blocks are no longer in use and can be wiped. When files are deleted, the operating system sends a TRIM command, marking these blocks as free for reuse.”

This rapidly becomes more complicated, though. For a start, the TRIM command for SATA doesn’t exist for NVMe, used by faster SSDs, where its closest substitute is DEALLOCATE. Neither is normally reported in the macOS log, although APFS does report its initial Trim when mounting an SSD. That’s reported for each container, not volume.

What we do know from often bitter experience is that some SSDs progressively slow down with use, a phenomenon most commonly (perhaps only?) seen with SATA drives connected over USB. Those also don’t get an initial Trim by APFS when they’re mounted.

It’s almost impossible to assess whether time required for Trim and housekeeping is likely to have any adverse effect on SSD write speed, provided that sufficient free disk space is maintained to support full-speed writing to the SLC write cache. Neither does there appear to be any need for a container to be remounted to trigger any Trim or housekeeping required to erase deleted storage ready for re-use, provided that macOS considers that SSD supports Trimming.

Getting best write performance from an SSD

• Avoid thermal throttling by keeping the SSD’s temperature controlled. For internal SSDs that needs active cooling by fans; for external SSDs that needs good enclosure design with cooling fins or possibly a fan.
• Keep ample free space on the SSD so the whole of its SLC write cache can be used.
• Limit continuous writes to within the SSD’s SLC write cache size, then allow sufficient time for the cache to empty before writing any more.
• It may be faster to use an SSD with a larger SLC write cache over a slower interface, than one with a smaller cache over a faster interface.
• Avoid SATA SSDs.

I’m grateful to Barry for raising these issues.

With more new M4 Macs in the offing, one question that I’m asked repeatedly is whether you should save money by getting a Mac with the smallest internal SSD and extend that using cheaper external storage. This article considers the pros and cons.

Size and prices

In Apple’s current M4 models, the smallest internal storage on offer is 256 GB. For the great majority, that’s barely adequate if you don’t install any of your own apps. It might suffice in some circumstances, for example if you work largely from shared storage, but for a standalone Mac it won’t be sufficient in five years time. Your starting point should therefore be a minimum of 512 GB internal SSD. Apple’s typical charge for increasing that to 2 TB is around $/€/£ 600.

The alternative to 2 TB internally would be an external 2 TB SSD. Unless you’re prepared to throw it away after three years, you’ll want to choose the most versatile interface that’s also backward compatible. The only choice here is Thunderbolt 5, which currently comes at a small premium over USB4 or Thunderbolt 3. Two TB would currently cost you $/€/£ 380-400, although those prices are likely to reduce in the coming months as TB5 SSDs come into greater supply.

Don’t be tempted to skimp with a USB 3.2 Gen 2 external SSD if that’s going to be your main storage. While it might seem a reasonable economy now, in 3-5 years time you’ll regret it. Besides, it may well have severe limitations in not Trimming as standard, and most don’t support SMART health indicators.

Thus, your expected saving by buying a Mac with only 512 GB internal storage, and providing 2 TB main storage on an external SSD, is around $/€/£ 200-220, and that’s really the only advantage in not paying Apple’s high price for an internal 2 TB SSD.

Upgrading internal storage in an Apple silicon model currently isn’t feasible for most users. As Apple doesn’t support such upgrades, they’re almost certain to invalidate its warranty and any AppleCare+ cover. That could change in the future, at least for some models like the Mac mini and Studio, but I think it unlikely that Apple would ever make an upgrade cheaper than initial purchase.

External boot disk

One of the few compelling reasons for choosing a Mac with minimal internal storage is when it’s going to be started up from an external boot disk. Because Apple silicon Macs must always start their boot process from their internal storage, and that Mac still needs Recovery and other features on its internal SSD, you can’t run entirely from an external SSD, but you could probably get away with the smallest available for its other specifications, either 256 or 512 GB.

Apple silicon Macs are designed to start up and run from their internal storage. Unlike Intel Macs with T2 chips, they will still boot from an external disk with Full Security, but there are several disadvantages in them doing so. Among those are the fact that, on an external boot disk, FileVault encryption isn’t performed in hardware and is inherently less secure, and AI isn’t currently supported when booted from an external disk. Choosing to do that thus involves compromises that you might not want to be stuck with throughout the lifetime of that Mac.

External media libraries

Regardless of the capacity of a Mac’s internal storage, it’s popular to store large media libraries on external storage, and for many that’s essential. This needs to be planned carefully: some libraries are easier to relocate than others, and provision has to be made for their backups. If you use hourly Time Machine backups for your working folders, you’ll probably want to back up external media libraries less frequently, and to different external storage.

External Home folder

Although it remains possible to relocate a user’s entire Home folder to external storage, this seems to have become more tricky in recent versions of macOS. Home folders also contain some of the most active files, particularly those in ~/Library, so moving them to an external SSD is going to require its good performance.

A more flexible alternative is to extend some working folders to external storage, while retaining the Home folder on internal storage. This can fit well with backup schedules, but you will still need to ensure the whole Home folder is backed up sufficiently frequently. This does have an unfortunate side-effect in privacy protection: this may require most of your working apps to be given access to Removable Volumes in the Files & Folders item in Privacy & Security settings. Thankfully, that should only need to be performed once when first using an app with external storage.

How much free space do you need?

When you’re weighing up your options to minimise the size of your new Mac’s internal storage, you also need to allow sufficient free space on each disk. APFS is very different from HFS+ in this respect: on external disks, in particular, HFS+ continues to work happily with just a few MB free, and could be filled almost to capacity. APFS, modern macOS and SSDs don’t work like that.

Measuring how much free space is needed isn’t straightforward either, as macOS trims back on its usage in response to falling free space. Some key features, such as retaining log entries, are sacrificed to allow others to continue. Snapshots can be removed or not made. Perhaps the best measurements come from observing the space requirements of VMs, where total virtual disk space much below 50 GB impairs running of normal functions. That’s the total size of the virtual disk, not the amount of free space, and doesn’t apply when iCloud or AI are enabled.

The other indicator of minimum free space requirements is for successful upgrading of macOS, which appears to be somewhere between 30-40 GB. This makes it preferable to keep an absolute minimum of around 50 GB free at all times. When possible, 100 GB gives more room for comfort.

SSD wear and performance

When the first M1 Macs were released, base models with just 8 GB of memory and 128 GB internal SSDs were most readily available, with custom builds (BTO) following later. As a result, many of those who set out to assess Apple’s new Macs ended up stress-testing those with inadequate memory and storage for the tasks they ran.

Many noticed rapid changes in their SSD wear indicators, and some were getting worryingly close to the end of their expected working life after just three years. Users also reported that SSD performance was falling. The reasons for those are that SSDs work best, age slowest, and remain fastest when they have ample free space. One common rule of thumb is to keep at least 20-25% of SSD capacity as free space, although evidence is largely empirical, and in places confused.

The simplest factor to understand is the effect of SSD size on wear. As the memory in an SSD is expected to last a fixed number of erase-write cycles, all other things being equal, writing and rewriting the same amount of data to a smaller SSD will reach that number more quickly. Thus, in general terms and under the same write load, a 512 GB SSD will last about half as long as a 1 TB SSD.

All other things aren’t equal, though, and that’s where wear levelling and Trim come into play. Without levelling the number of erase-write cycles across all the memory in an SSD, some would reach their limit far sooner than others. To tackle that, SSDs incorporate mechanisms to even out the use of individual memory cells, as wear levelling. The less free space available on an SSD, the less effective wear levelling can be, giving larger SSDs a significant advantage if they also have more free space.

Trimming is performed periodically to allow storage that has already been made available for reuse, for example when a file has been deleted, to be erased and made ready. Both APFS and HFS+ will Trim compatible SSDs when mounting a volume, but Trim support for external SSDs is only provided by default for those with NVMe interfaces, not SATA, and isn’t available for other file systems including ExFAT. Some SSDs may still be able to process available storage in their routine housekeeping, but others won’t. Without Trimming, an SSD gradually fills with unused memory waiting to be erased, and will steadily grind to a halt, with write speeds falling to about 10% of new.

Thus, to ensure optimum performance and working life, SSDs should be as large as possible, with much of their storage kept free. Experience suggests that a healthy amount of free space is 20-50% of their capacity.

Striking the best compromise

Apple silicon Macs work best and fastest when largely running from their internal SSDs. By all means reduce the capacity required by moving more static media libraries, and possibly large working folders, to an external SSD. But there’s no escaping the evidence that your Mac will work best and longest when its internal storage has a minimum of 20% free at all times, and you must ensure that never falls below 50 GB free space. Finally, consider your needs not today, but when you intend replacing that Mac in 3-5 years time, or any savings made now will prove a false economy.