In my /etc/fstab file I have an entry for my swap as follows:
/root/swap swap swap sw 0 0
I have other machines and also I've seen online that sometimes they put default or xfs or other options. Then, I'm a little confused on what 'sw' means and what's for, and also which one would be the best option to put there and why.
From the fstab manual on my system:
The fourth field,
fs_mntops, describes the mount options associated with the filesystem. It is formatted as a comma separated list of options. It contains at least the type of mount (seefs_typebelow) plus any additional options appropriate to the filesystem type. [...]If
fs_typeis “rw”, “rq”, or “ro” then the filesystem whose name is given in thefs_filefield is normally mounted read-write or read-only on the specified special file. Iffs_typeis “sw” then the special file is made available as a piece of swap space by theswapon(8)command at the end of the system reboot procedure.
So basically, sw is used to tell swapon (or swapctl on my system) that this is a valid candidate for use as swap space that will be added as part of the system start-up routine.
From the manual describing swapctl -A:
This option causes
swapctlto read the/etc/fstabfile for devices and files with an “sw” type, and adds all these entries as swap devices. If no swap devices are configured,swapctlwill exit with an error code.
That's on OpenBSD. On the Ubuntu Linux system that I have access to, neither manual mentions sw as a mount option for swap for some reason.
sw option doesn't do anything, and is simply a placeholder (b) tells swapon to activate the swap space. But why this should be necessary, I don't know, since the space is already specified as swap by the filesystem type. In any case, in view of the apparently wide usage of sw on Linux, this probably represents a lacuna in the documentation, and should probably be reported.
Commented
May 18, 2017 at 20:18
sw may be changed to xx to explicitly disable a swap partition. So it's not completely useless. Again, that's from the fstab manual on OpenBSD.
If fs_type is “sw”. The fourth column isn't fs_type, it's mount options.
Commented
May 18, 2017 at 20:39
fs_mntops). These options contains the fs_type, the type of mount. The type is read-write (rw and rq (quota)), read-only (ro), or swap (sw) (or xx for disabled).
For Linux, at least, based on a look at the net, this appears to be an example of "cargo culting", which means including code or words in this case for ritualistic or superstitions reasons, even though they may "serve no real purpose." The evidence I've seen suggests that this option isn't actually necessary or meaningful, but you need to put something there, because the fstab file syntax expects it. You can easily find examples of people using defaults and even just pri=1 or whatever, leaving out both defaults and sw.
And, it doesn't make sense that swapon would need to look at the mount options to check that it is swap, because swapon can already see it's swap by looking at the filesystem type. There are certainly mount options for swap that modify behavior, as documented in man swapon, but sw isn't one of them. So the evidence is that it is a placeholder, and foobar would do just as well as any other placeholder.
I can't be completely sure of this without looking at the source code of course.
Related: a Debian bug report complaining about this exact issue: mount: swapon(8) lacks explanation for sw and defaults options.
I just added this to my answer here: Ask Ubuntu: How do I increase the size of swapfile without removing it in the terminal?:
For the meaning of
swin the fourth field of the/etc/fstabfile, on BSD systems it meansswapon, and on Linux it means nothing at all and is not documented anywhere, but acts as a "cargo-culted" superstitious and meaningless placeholder for historical reasons. Since having it there ensures this fstab (Filesystem Table) entry is compatible with BSD systems, and since it won't break Linux systems, it's a good idea to include it for maximum compatibility.
