PMDAJBD2(1)

💖 PMDAJBD2 💖

okay bestie, welcome to the General Commands Manual section, page PMDAJBD2(1) — yes that's the whole title, she's long, she's iconic, she's a whole vibe 💅

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✨ NAME ✨

pmdajbd2 — the journal block device (JBD) performance metrics domain agent (PMDA). she watches the journal. she reports the numbers. that's the job 🎀

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🌸 SYNOPSIS 🌸

$PCP_PMDAS_DIR/jbd2/pmdajbd2 [-D debug] [-d domain] [-l logfile] [-j
path] [-U username]

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🦋 DESCRIPTION 🦋

so pmdajbd2 is a Performance Metrics Domain Agent (PMDA), and what she does is pull performance metrics out of the Journal Block Device subsystem (version 2) inside the Linux kernel. lowkey a big deal. the kernel hands these metrics out through procfs files, and there's like one file per block device, which is very organized of her fr fr.

the JBD2 subsystem is used by a bunch of filesystems, including ext3, ext4 and ocfs2 — so if you're running any of those, this is your girl 🌈

the jbd2 PMDA exports metrics that measure detailed journal transaction information, like time spent waiting and locked, request rates, blocks used, and so on. it's giving very thorough. obsessed. ✨

here's the quick rundown of the pmdajbd2 command line options, and just so we're clear — these are only relevant when running the PMDA as a daemon, not as a shared library. iykyk 🫶

• -d — the performance metrics domain number you put here has to be unique and consistent, and honestly this is absolutely crucial, no cap. what that means: domain should be different for every PMDA on the one host, and the same domain number should be used for the same PMDA on all hosts. consistency is the moment 💜
• -l — where the log file lives. by default, when running as a daemon, a log file named jbd2.log gets written in the current directory from when pmdajbd2 is started, i.e. $PCP_LOG_DIR/pmcd. if the log file can't be created or isn't writable, output goes to standard error instead. and when running in shared library mode, diagnostic information gets written into the pmcd log file, namely $PCP_LOG_DIR/pmcd/pmcd.log. she adapts 💅
• -j — lets you specify an alternate path to the jbd2 statistics files. the default path is /proc/fs/jbd2. custom paths? we love to see it 🎀
• -U — the user account to run the agent under. the default is the unprivileged "pcp" account in current versions of PCP, but in older versions the superuser account ("root") was used by default. glow-up behavior honestly 😭

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🌟 INSTALLATION 🌟

this PMDA is installed by default, and in shared library mode (rather than as a separate daemon to pmcd(1)). which means the names, help text and values for the jbd2 performance metrics should always be available. always on, always serving 💖

if you don't use these metrics and you want to remove this PMDA, do the following as root:

# cd $PCP_PMDAS_DIR/jbd2
# ./Remove

and if you want to enable the installation again, do the following as root:

# cd $PCP_PMDAS_DIR/jbd2
# ./Install

pmdajbd2 is launched by pmcd(1) and should never be executed directly, okay?? do not do it. main character does not walk herself onto the stage 🙈 the Install and Remove scripts notify pmcd(1) when the agent is installed or removed, so the whole system stays in the loop. periodt.

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🍓 FILES 🍓

• $PCP_PMCDCONF_PATH — command line options used to launch pmdajbd2
• $PCP_PMDAS_DIR/jbd2/help — default help text file for the jbd2 metrics
• $PCP_PMDAS_DIR/jbd2/Install — installation script for the pmdajbd2 agent
• $PCP_PMDAS_DIR/jbd2/Remove — undo installation script for the pmdajbd2 agent
• $PCP_LOG_DIR/pmcd/jbd2.log — default log file for error messages and other information from pmdajbd2

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🌈 PCP ENVIRONMENT 🌈

environment variables with the prefix PCP_ are used to parameterize the file and directory names used by PCP. on each installation, the file /etc/pcp.conf contains the local values for these variables. and the $PCP_CONF variable may be used to specify an alternative configuration file, as described in pcp.conf(5). configurable, flexible, gorgeous 💜

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🧚 DEBUGGING OPTIONS 🧚

the -D or --debug option enables the output of additional diagnostics on stderr to help triage problems — though ngl, the information is sometimes cryptic and primarily intended to provide guidance for developers rather than for end-users. so if you're confused, that's not a you problem, it's a feature 💀

debug is a comma separated list of debugging options. use pmdbg(1) with the -l option to obtain a list of the available debugging options and their meaning. go get your list bestie, she's right there ✨

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👑 SEE ALSO 👑

PCPIntro(1), pmcd(1), pcp.conf(5) and pcp.env(5).

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okay that's the whole page, you ate it, no crumbs 🫶

Performance Co-Pilot 🎀 PCP 🎀 and yes, we were on the PMDAJBD2(1) page the entire time — slay, bye bestie 💖✨