Why SSDs slow down, and how to avoid it
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.
