zfs_arc plugin skips hits/misses in collectd5.9.2g on 20.04

[Durk van Veen]

$ lsb_release -rd
Description: Ubuntu 20.04 LTS
Release: 20.04
$ dpkg --list | grep collectd
ii collectd-core 5.9.2.g-1ubuntu5 amd64 statistics collection and monitoring daemon (core system)

The zfs_arc plug-in for collectd is broken in 5.9.2g for the "hits" and "misses" lines of /proc/spl/kstat/zfs/arcstats. Due to a bug in zfs_arc.c, the first 2 lines of this file are skipped twice, resulting in 4 lines being skipped.

I was able to confirm that just by taking /usr/lib/collectd/zfs_arc.so from a working 18.04 installation and dropping it on the 20.04 box, the problem could be resolved.

Then I rebuilt the package from source with a single hunk of code removed from zfs_arc.c that was not present in 5.9.2, introduced in 5.9.2g, and removed again in upstream debian 5.10.0. This fixes the issue.

I also traced it to the collectd upstream and there the change was made as part of commit 0cc3044f with a commit message that specifically describes the problem.

--- collectd.5.9.2g/zfs_arc.c 2020-04-29 23:19:26.307645579 -0700
+++ collectd.5.10.0/zfs_arc.c 2020-04-29 23:20:19.658663146 -0700
@@ -231,23 +231,6 @@
     return -1;
   }

- // Ignore the first two lines because they contain information about
- // the rest of the file.
- // See kstat_seq_show_headers module/spl/spl-kstat.c of the spl kernel
- // module.
- if (fgets(buffer, sizeof(buffer), fh) == NULL) {
- ERROR("zfs_arc plugin: \"%s\" does not contain a single line.",
- ZOL_ARCSTATS_FILE);
- fclose(fh);
- return (-1);
- }
- if (fgets(buffer, sizeof(buffer), fh) == NULL) {
- ERROR("zfs_arc plugin: \"%s\" does not contain at least two lines.",
- ZOL_ARCSTATS_FILE);
- fclose(fh);
- return (-1);
- }
-
   while (fgets(buffer, sizeof(buffer), fh) != NULL) {
     char *fields[3];
     value_t v;