Why are swap partitions discouraged on SSD drives, are they harmful?

I often read that one should not place swap partitions on a SSD drive, as this may harm the device. Is this true? Can you please explain the reason to me?

Because I otherwise would have thought that placing swap on an SSD is the best choice, as it's much faster than HDDs and therefore swapping RAM contents to the SSD is not as slow as it would be with the HDD...

Flash RAM cells in SSDs have a limited lifespan. Every write (but not read) cycle (or more accurately every erasure) wears a memory cell, and at some point it will stop working.

The amount of erase cycles a cell can survive is highly variable, and flash from modern SSDs will survive many more than flash from SSDs made several years ago. Additionally, the SSD intelligent firmware will ensure evenly distributed erasures between all the cells. In most drives, unused areas will also be available to backup damaged cells and to delay aging.

To have a value we can use to compare the endurance of a SSD, we can use lifespan measures such as the JEDEC published standards. A widely available value for endurance is TBW (TeraBytes Written, or alternatively total bytes written) which is the amount of bytes writeable before the drive fails. Modern SSDs can score as low as 20 TB for a consumer product but can score over 20,000 TB in an enterprise-level SSD.

Having said that, both the lifespan and the use of a SSD for swapping depends on several factors...

Systems with plenty of RAM

On a system with plenty of RAM and few memory consuming applications we will almost never swap. It is merely a safety measure to prevent data loss in case an application ate up all our RAM. In this case, the wearing of a SSD from swapping will not be an issue. However, having this mostly-unused swap partition on a conventional hard drive will not lead to any performance drop, so we can safely put our swap partition (or file) on that significantly cheaper hard drive and use the space on our SSD for something more useful.

Systems with little RAM

Things are different on a system where RAM is sparse and cannot be upgraded. In this case, swapping may indeed occur more often, especially when we run memory-intensive applications. In these systems, a swap partition or file on a SSD may lead to a dramatic performance improvement at the cost of a somewhat shorter SSD lifespan. This decreased lifespan may, however, still not be short enough to warrant concern. In all likelihood, the SSD may be replaced long before it would've died because several times the storage may be available at a fraction of today's prices.

Hibernating our system

Waking from hibernation is indeed very fast from a SSD. If we're lucky and our system survives a hibernation without issues, we can consider using an SSD for that. It will wear the SSD more than just booting from it would, but we may feel it's worth it.

But booting from an SSD may not take much longer than waking from hibernation from an SSD, and it will wear the SSD far less. Personally, I don't hibernate my system at all - I suspend to RAM or quickly boot from my SSD.

The SSD is the only drive we have

We don't really have a choice in this case. We don't want to run without a swap, so we have to put it on the SSD. We may, however, want to have a smaller swap file or partition if we don't plan to hibernate our system at any point.

Note on speed

SSDs are best at quickly accessing and reading many small files and are superior to conventional hard drives for transferring data from sequentially-read small or medium-sized files. A fast conventional hard drive may still perform better than an SSD at writing (and to a lesser extent reading) large audio or video streams or other long unfragmented files. Older SSDs may have their performance decline over time or after they are fairly full.

Early SSDs had a reputation for failing after fewer writes than HDDs. If the swap was used often, then the SSD may fail sooner. This might be why you heard it could be bad to use an SSD for swap.

Modern SSDs don't have this issue, and they should not fail any faster than a comparable HDD. Placing swap on an SSD will result in better performance than placing it on an HDD due to its faster speeds.

Additionally, if your system has enough RAM (likely, if the system is high-end enough to have an SSD), the swap may be used only rarely anyway.

Even if you have enough RAM, you might still want to prevent any file copy or search to swap out the applications from RAM. This might be the case on file servers (NAS, SAMBA, FTP) which might be involved in large file operations.

In order to do that it's best to set in /etc/sysctl.conf:

vm.swappiness=1
vm.vfs_cache_pressure=50

The first setting prevents disk cache (e.g. doing cp) from swapping out existing apps from RAM. The normal default setting on that is 60. Note that using 0, although more aggressive, has been sometimes reported to generate out-of-memory errors.

The second setting prevents file searches (e.g. doing find) from swapping out existing apps from RAM. The normal default setting for that is 100.

Although the author mentioned in reference does not refer explicitly to SSD's, this approach also reduces wear on SSD due to reduced swapping and he also provides example how to test it.

Reference: https://rudd-o.com/linux-and-free-software/tales-from-responsivenessland-why-linux-feels-slow-and-how-to-fix-that