Update swapon.8 to discourage using ZFS as swap device/file.

[ota_j.email.ne.jp]

Summary

ZFS swap space can trigger OOM much earlier.

Test Plan

man swapon

[bcr]

Two suggestions.

sbin/swapon/swapon.8

211

killing what? I would insert "(random?) processes" here.
Contractions are discouraged in man pages, so use "does not" here.

[kib]

sbin/swapon/swapon.8

208

It does not matter how much memory is required for write. What matter is that each write requires allocation of memory.

[debdrup]

If the VM high watermark levels are tweaked, OOM from ZFS on swap can be entirely avoided. Don't know if this is relevant?

[bcr]

Any updates on this one? The suggested changes don't seem too difficult to me.
@ota_j.email.ne.jp are you interested in finishing this?

[ota_j.email.ne.jp]

I've completely forgotten this changeset.

How does it look this time?

[rpokala]

.Sh NOTE
ZVOLs are discouraged for use as swap devices,
because ZFS needs to allocate memory in order to write,
which may not be possible while the system is under memory pressure.
In other words, ZFS needs memory to make memory available.
ZFS swap files are discouraged for the same reason.

[kib]

In D22150#836604, @rpokala wrote:

.Sh NOTE
ZVOLs are discouraged for use as swap devices,
because ZFS needs to allocate memory in order to write,
which may not be possible while the system is under memory pressure.
In other words, ZFS needs memory to make memory available.
ZFS swap files are discouraged for the same reason.

Why don't you go ahead and commit this, referencing original submitter in credits?

[ota_j.email.ne.jp]

Thanks for revising.

[pauamma_gundo.com]

Minor, fixable on commit.

sbin/swapon/swapon.8

212

s/OOM/The OOM/

213

Missing word here? "or fails to allocate" maybe?

[grahamperrin]

Upstream:

• zfs-create(8): ZFS for swap: caution, grammar by grahamperrin · Pull Request #14756 · openzfs/zfs.

I opened this PR in response to https://github.com/openzfs/zfs/issues/7734#issuecomment-1510451126; I wasn't aware of D22150 at the time.