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zed: Add synchronous zedlets

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Historically, ZED has blindly spawned off zedlets in parallel and never
worried about their completion order.  This means that you can
potentially have zedlets for event number 2 starting before zedlets for
event number 1 had finished.  Most of the time this is fine, and it
actually helps a lot when the system is getting spammed with hundreds
of events.

However, there are times when you want your zedlets to be executed
in sequence with the event ID.  That is where synchronous zedlets
come in.

ZED will wait for all previously spawned zedlets to finish before
running a synchronous zedlet.  Synchronous zedlets are guaranteed to be
the only zedlet running.  No other zedlets may run in parallel with a
synchronous zedlet.  Users should be careful to only use synchronous
zedlets when needed, since they decrease parallelism.

To make a zedlet synchronous, simply add a "-sync-" immediately
following the event name in the zedlet's file name:

	EVENT_NAME-sync-ZEDLETNAME.sh

For example, if you wanted a synchronous statechange script:

	statechange-sync-myzedlet.sh

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #17335
This commit is contained in:
Tony Hutter
2025-09-11 11:34:07 -07:00
committed by GitHub
parent bc0b5318aa
commit 654f2dcb42
15 changed files with 289 additions and 37 deletions
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man/man8/zed.8.in
+32
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@@ -158,6 +158,8 @@ Multiple ZEDLETs may be invoked for a given zevent.
ZEDLETs are executables invoked by the ZED in response to a given zevent.
They should be written under the presumption they can be invoked concurrently,
and they should use appropriate locking to access any shared resources.
The one exception to this are "synchronous zedlets", which are described later
in this page.
Common variables used by ZEDLETs can be stored in the default rc file which
is sourced by scripts; these variables should be prefixed with
.Sy ZED_ .
@@ -233,6 +235,36 @@ and
.Sy ZPOOL .
These variables may be overridden in the rc file.
.
.Sh Synchronous ZEDLETS
ZED's normal behavior is to spawn off zedlets in parallel and ignore their
completion order.
This means that ZED can potentially
have zedlets for event ID number 2 starting before zedlets for event ID number
1 have finished.
Most of the time this is fine, and it actually helps when the system is getting
hammered with hundreds of events.
.Pp
However, there are times when you want your zedlets to be executed in sequence
with the event ID.
That is where synchronous zedlets come in.
.Pp
ZED will wait for all previously spawned zedlets to finish before running
a synchronous zedlet.
Synchronous zedlets are guaranteed to be the only
zedlet running.
No other zedlets may run in parallel with a synchronous zedlet.
Users should be careful to only use synchronous zedlets when needed, since
they decrease parallelism.
.Pp
To make a zedlet synchronous, simply add a "-sync-" immediately following the
event name in the zedlet's file name:
.Pp
.Sy EVENT_NAME-sync-ZEDLETNAME.sh
.Pp
For example, if you wanted a synchronous statechange script:
.Pp
.Sy statechange-sync-myzedlet.sh
.
.Sh FILES
.Bl -tag -width "-c"
.It Pa @sysconfdir@/zfs/zed.d