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Differential D22150

Update swapon.8 to discourage using ZFS as swap device/file.
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Authored by ota_j.email.ne.jp on Oct 25 2019, 3:37 AM.
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alc
markj
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dougm
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manpages
Summary

ZFS swap space can trigger OOM much earlier.

Test Plan

man swapon

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rS FreeBSD src repository - subversion
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Buildable 27215
Build 25480: arc lint + arc unit

Event Timeline

ota_j.email.ne.jp created this revision.Oct 25 2019, 3:37 AM
Herald added a reviewer: manpages. · View Herald TranscriptOct 25 2019, 3:37 AM
Herald added a subscriber: imp. · View Herald Transcript
Harbormaster completed remote builds in B27215: Diff 63660.Oct 25 2019, 3:37 AM
bcr added a subscriber: bcr.Oct 25 2019, 5:59 AM

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 added inline comments.Oct 25 2019, 6:51 AM
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 added a subscriber: debdrup.Oct 26 2019, 9:43 AM

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

gbe added a subscriber: gbe.Jan 9 2021, 1:54 PM
bcr added a comment.Oct 1 2022, 9:06 AM

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 updated this revision to Diff 111331.Oct 3 2022, 2:56 AM

I've completely forgotten this changeset.

How does it look this time?

rpokala added a subscriber: rpokala.Oct 3 2022, 5:14 AM
.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.
dougm resigned from this revision.Oct 3 2022, 7:18 AM
kib added a comment.Oct 3 2022, 10:36 PM
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 added a comment.Oct 4 2022, 2:31 AM

Thanks for revising.

pauamma_gundo.com accepted this revision.Oct 6 2022, 8:04 AM
pauamma_gundo.com added a subscriber: 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?

This revision is now accepted and ready to land.Oct 6 2022, 8:04 AM
grahamperrin added a subscriber: grahamperrin.Apr 23 2023, 5:33 AM

Upstream:

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.

Revision Contents
Changeset List

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sbin/
swapon/
9 lines

Diff 63660

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sbin/swapon/swapon.8

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