Linux Swap – Is ‘Page-to-Disk’ Feature the Same as Swapping?

Don't confuse (the) swap (as a disk area) and (to) swap (as a method to move memory pages from RAM to disk and reciprocally).

Excessive swapping is something to be avoided for performance reasons but having a swap area isn't necessarily a problem.

On systems, like Linux, that overcommit memory, i.e. that allow processes to allocate more memory than available, running out of RAM with not enough swap to handle the situation will trigger the OOM killer. You have to trust the algorithm used to select the "right" process to kill, and accept one or more of your processes to be killed without being given a chance to shut down properly. Here is a famous analogy that explain why the OOM killer might not be a good idea at all.

On systems like Solaris, that do not overcommit memory, i.e that make sure a memory reservation is always backed by virtual memory, whether in RAM or on disk, having a sufficient swap area is absolutely necessary otherwise a potentially significant part of the RAM will be wasted.

We trust the integrity of the data retrieved from swap because the storage hardware has checksums, CRCs, and such.

In one of the comments above, you say:

true, but it won't protect against bit flips outside of the disk itself

"It" meaning the disk's checksums here.

That is true, but SATA uses 32-bit CRCs for commands and data. Thus, you have a 1 in 4 billion chance of corrupting data undetectably between the disk and the SATA controller. That means that a continuous error source could introduce an error as often as every 125 MiB transferred, but a rare, random error source like cosmic rays would cause undetectable errors at a vanishingly small rate.

Realize also that if you've got a source that causes an undetected error at a rate anywhere near one per 125 MiB transferred, performance will be terrible because of the high number of detected errors requiring re-transfer. Monitoring and logging will probably alert you to the problem in time to avoid undetected corruption.

As for the storage medium's checksums, every SATA (and before it, PATA) disk uses per-sector checksums of some kind. One of the characteristic features of "enterprise" hard disks is larger sectors protected by additional data integrity features, greatly reducing the chance of an undetected error.

Without such measures, there would be no point to the spare sector pool in every hard drive: the drive itself could not detect a bad sector, so it could never swap fresh sectors in.

In another comment, you ask:

if SATA is so trustworthy, why are there checksummed file systems like ZFS, btrfs, ReFS?

Generally speaking, we aren't asking swap to store data long-term. The limit on swap storage is the system's uptime, and most data in swap doesn't last nearly that long, since most data that goes through your system's virtual memory system belongs to much shorter-lived processes.

On top of that, uptimes have generally gotten shorter over the years, what with the increased frequency of kernel and libc updates, virtualization, cloud architectures, etc.

Furthermore, most data in swap is inherently disused in a well-managed system, being one that doesn't run itself out of main RAM. In such a system, the only things that end up in swap are pages that the program doesn't use often, if ever. This is more common than you might guess. Most dynamic libraries that your programs link to have routines in them that your program doesn't use, but they had to be loaded into RAM by the dynamic linker. When the OS sees that you aren't using all of the program text in the library, it swaps it out, making room for code and data that your programs are using. If such swapped-out memory pages are corrupted, who would ever know?

Contrast this with the likes of ZFS where we expect the data to be durably and persistently stored, so that it lasts not only beyond the system's current uptime, but also beyond the life of the individual storage devices that comprise the storage system. ZFS and such are solving a problem with a time scale roughly two orders of magnitude longer than the problem solved by swap. We therefore have much higher corruption detection requirements for ZFS than for Linux swap.

ZFS and such differ from swap in another key way here: we don't RAID swap filesystems together. When multiple swap devices are in use on a single machine, it's a JBOD scheme, not like RAID-0 or higher. (e.g. macOS's chained swap files scheme, Linux's swapon, etc.) Since the swap devices are independent, rather than interdependent as with RAID, we don't need extensive checksumming because replacing a swap device doesn't involve looking at other interdependent swap devices for the data that should go on the replacement device. In ZFS terms, we don't resilver swap devices from redundant copies on other storage devices.

All of this does mean that you must use a reliable swap device. I once used a $20 external USB HDD enclosure to rescue an ailing ZFS pool, only to discover that the enclosure was itself unreliable, introducing errors of its own into the process. ZFS's strong checksumming saved me here. You can't get away with such cavalier treatment of storage media with a swap file. If the swap device is dying, and is thus approaching that worst case where it could inject an undetectable error every 125 MiB transferred, you simply have to replace it, ASAP.

The overall sense of paranoia in this question devolves to an instance of the Byzantine generals problem. Read up on that, ponder the 1982 date on the academic paper describing the problem to the computer science world, and then decide whether you, in 2019, have fresh thoughts to add to this problem. And if not, then perhaps you will just use the technology designed by three decades of CS graduates who all know about the Byzantine Generals Problem.

This is well-trod ground. You probably can't come up with an idea, objection, or solution that hasn't already been discussed to death in the computer science journals.

SATA is certainly not utterly reliable, but unless you are going to join academia or one of the the kernel development teams, you are not going to be in a position to add materially to the state of the art here. These problems are already well in hand, as you've already noted: ZFS, btrfs, ReFS... As an OS user, you simply have to trust that the OS's creators are taking care of these problems for you, because they also know about the Byzantine Generals.

It is currently not practical to put your swap file on top of ZFS or Btrfs, but if the above doesn't reassure you, you could at least put it atop xfs or ext4. That would be better than using a dedicated swap partition.