The more blog posts I write the harder it becomes to come up with a good title. So what is this post all about? In short, it’s about the implications of using systemd to start a single instance Oracle database in Oracle Linux and associated shell limits. In theory this should apply to Single Instance Oracle only – Oracle Restart/Real Application Clusters use different means to control the start of databases via their respective registries.
A short introduction to systemd
As of Oracle Linux 7 systemd replaced upstart in the same way upstart replaced SysV init earlier. systemd is a system and service manager for Linux Operating systems and does a lot more than either of its predecessors. It’s also extremely well documented in man pages.
An Oracle administrator might use systemd to start a database automatically when a system boots up. This is done via a so-called unit file, containing the necessary instructions to start the Oracle database.
Which leads me back to the topic of this post. You can read at oracle-base.com and a few other places that systemd does not respect shell limits set by pam_limits(8). Which is considered a feature; feel free to check the bugzilla discussion for more details. Remember that pam_limits(8) and its associated limits.conf file is used to configure shell limits for the oracle user, as per the Database Installation Guide.
To demonstrate the effect I’m going to use my most recent Oracle Linux 8.4 lab VM. The oracle account has been created and configured via the 19c preinstall RPM. I’m using my Vagrant box, updated to Oracle Linux 8.4 including all patches up to June 14th.
NOTE: the settings you’ll see later in the unit files are for Oracle Linux 8.4. Although Oracle Linux 7 uses systemd as well it might not support the same directives and/or syntax.
Let’s have a look at systemd unit files.
Creating a oneshot systemd unit file
I picked /etc/systemd/system as the location for my new unit file. At least in Oracle Linux 8.4 that seems to be the preferred location for custom unit files.
WARNING: This is totally lab VM/playground territory, you should never do this in a real (live) environment!
If you find the examples confusing because you aren’t familiar with systemd please head over to the documentation for more information.
# cat /etc/systemd/system/limitstest.service [Unit] Description=A oneshot service to test whether systemd respects PAM limits [Service] User=oracle Group=oinstall Type=oneshot ExecStart="/home/oracle/test.sh"
This unit file merely fires off /home/oracle/test.sh (once) as oracle:oinstall. The little shell script in /home/oracle/test.sh couldn’t be simpler:
$ cat /home/oracle/test.sh #!/usr/bin/env bash echo "-----------------------------------------------------------" echo "test run starts at $(/usr/bin/date):" echo "I am $(/usr/bin/whoami)" echo "" echo "my own PID is: $$" echo "" /usr/bin/ps -ef| /usr/bin/grep $$ | /usr/bin/grep -v grep echo "" echo "hard limits according to ulimit:" echo "" ulimit -Ha echo "" echo "soft limits according to ulimit:" echo "" ulimit -Sa echo "" echo "actual limits as per /proc": echo "" /usr/bin/cat /proc/$$/limits echo -----------------------------------------------------------
It prints a few PIDs and shell limits (both hard and soft limits). Finally, it looks into the /proc file system to get the shell limits of itself (${$}).
Unless you restart your (virtual) server, systemd won’t know about the new unit file. Alternatively you can run systemctl daemon-reload to reload the daemon.
Running the unit file
So let’s go ahead and see what the output of the unit file is. I started it using systemctl start limitstest.service.
# journalctl -u limitstest.service ... Jun 15 19:44:07 server1 systemd[1]: Starting A oneshot service to test whether systemd respects PAM limits... Jun 15 19:44:07 server1 test.sh[13046]: ----------------------------------------------------------- Jun 15 19:44:07 server1 test.sh[13046]: test run starts at Tue Jun 15 19:44:07 UTC 2021: Jun 15 19:44:07 server1 test.sh[13046]: I am oracle Jun 15 19:44:07 server1 test.sh[13046]: my own PID is: 13046 Jun 15 19:44:07 server1 test.sh[13046]: oracle 13046 1 0 19:44 ? 00:00:00 bash /home/oracle/test.sh Jun 15 19:44:07 server1 test.sh[13046]: oracle 13049 13046 0 19:44 ? 00:00:00 /usr/bin/ps -ef Jun 15 19:44:07 server1 test.sh[13046]: hard limits according to ulimit: Jun 15 19:44:07 server1 test.sh[13046]: core file size (blocks, -c) unlimited Jun 15 19:44:07 server1 test.sh[13046]: data seg size (kbytes, -d) unlimited Jun 15 19:44:07 server1 test.sh[13046]: scheduling priority (-e) 0 Jun 15 19:44:07 server1 test.sh[13046]: file size (blocks, -f) unlimited Jun 15 19:44:07 server1 test.sh[13046]: pending signals (-i) 30554 Jun 15 19:44:07 server1 test.sh[13046]: max locked memory (kbytes, -l) 64 Jun 15 19:44:07 server1 test.sh[13046]: max memory size (kbytes, -m) unlimited Jun 15 19:44:07 server1 test.sh[13046]: open files (-n) 262144 Jun 15 19:44:07 server1 test.sh[13046]: pipe size (512 bytes, -p) 8 Jun 15 19:44:07 server1 test.sh[13046]: POSIX message queues (bytes, -q) 819200 Jun 15 19:44:07 server1 test.sh[13046]: real-time priority (-r) 0 Jun 15 19:44:07 server1 test.sh[13046]: stack size (kbytes, -s) unlimited Jun 15 19:44:07 server1 test.sh[13046]: cpu time (seconds, -t) unlimited Jun 15 19:44:07 server1 test.sh[13046]: max user processes (-u) 30554 Jun 15 19:44:07 server1 test.sh[13046]: virtual memory (kbytes, -v) unlimited Jun 15 19:44:07 server1 test.sh[13046]: file locks (-x) unlimited Jun 15 19:44:07 server1 test.sh[13046]: soft limits according to ulimit: Jun 15 19:44:07 server1 test.sh[13046]: core file size (blocks, -c) unlimited Jun 15 19:44:07 server1 test.sh[13046]: data seg size (kbytes, -d) unlimited Jun 15 19:44:07 server1 test.sh[13046]: scheduling priority (-e) 0 Jun 15 19:44:07 server1 test.sh[13046]: file size (blocks, -f) unlimited Jun 15 19:44:07 server1 test.sh[13046]: pending signals (-i) 30554 Jun 15 19:44:07 server1 test.sh[13046]: max locked memory (kbytes, -l) 64 Jun 15 19:44:07 server1 test.sh[13046]: max memory size (kbytes, -m) unlimited Jun 15 19:44:07 server1 test.sh[13046]: open files (-n) 1024 Jun 15 19:44:07 server1 test.sh[13046]: pipe size (512 bytes, -p) 8 Jun 15 19:44:07 server1 test.sh[13046]: POSIX message queues (bytes, -q) 819200 Jun 15 19:44:07 server1 test.sh[13046]: real-time priority (-r) 0 Jun 15 19:44:07 server1 test.sh[13046]: stack size (kbytes, -s) 8192 Jun 15 19:44:07 server1 test.sh[13046]: cpu time (seconds, -t) unlimited Jun 15 19:44:07 server1 test.sh[13046]: max user processes (-u) 30554 Jun 15 19:44:07 server1 test.sh[13046]: virtual memory (kbytes, -v) unlimited Jun 15 19:44:07 server1 test.sh[13046]: file locks (-x) unlimited Jun 15 19:44:07 server1 test.sh[13046]: actual limits as per /proc: Jun 15 19:44:07 server1 test.sh[13046]: Limit Soft Limit Hard Limit Units Jun 15 19:44:07 server1 test.sh[13046]: Max cpu time unlimited unlimited seconds Jun 15 19:44:07 server1 test.sh[13046]: Max file size unlimited unlimited bytes Jun 15 19:44:07 server1 test.sh[13046]: Max data size unlimited unlimited bytes Jun 15 19:44:07 server1 test.sh[13046]: Max stack size 8388608 unlimited bytes Jun 15 19:44:07 server1 test.sh[13046]: Max core file size unlimited unlimited bytes Jun 15 19:44:07 server1 test.sh[13046]: Max resident set unlimited unlimited bytes Jun 15 19:44:07 server1 test.sh[13046]: Max processes 30554 30554 processes Jun 15 19:44:07 server1 test.sh[13046]: Max open files 1024 262144 files Jun 15 19:44:07 server1 test.sh[13046]: Max locked memory 65536 65536 bytes Jun 15 19:44:07 server1 test.sh[13046]: Max address space unlimited unlimited bytes Jun 15 19:44:07 server1 test.sh[13046]: Max file locks unlimited unlimited locks Jun 15 19:44:07 server1 test.sh[13046]: Max pending signals 30554 30554 signals Jun 15 19:44:07 server1 test.sh[13046]: Max msgqueue size 819200 819200 bytes Jun 15 19:44:07 server1 test.sh[13046]: Max nice priority 0 0 Jun 15 19:44:07 server1 test.sh[13046]: Max realtime priority 0 0 Jun 15 19:44:07 server1 test.sh[13046]: Max realtime timeout unlimited unlimited us Jun 15 19:44:07 server1 test.sh[13046]: ----------------------------------------------------------- Jun 15 19:44:07 server1 systemd[1]: limitstest.service: Succeeded. Jun 15 19:44:07 server1 systemd[1]: Started A oneshot service to test whether systemd respects PAM limits. ...
All right, that worked and we are off to a good start. Those of us who installed Oracle a few times might already spot a few details in the above output. It’s not quite what I had in mind.
Executing test.sh in an interactive shell
For comparison, this is the output when logged in as oracle (in an interactive shell)
$ ./test.sh ----------------------------------------------------------- test run starts at Tue Jun 15 20:13:41 UTC 2021: I am oracle my own PID is: 13173 oracle 13173 13150 0 20:13 pts/0 00:00:00 bash ./test.sh oracle 13176 13173 0 20:13 pts/0 00:00:00 /usr/bin/ps -ef hard limits according to ulimit: core file size (blocks, -c) unlimited data seg size (kbytes, -d) unlimited scheduling priority (-e) 0 file size (blocks, -f) unlimited pending signals (-i) 30554 max locked memory (kbytes, -l) 134217728 max memory size (kbytes, -m) unlimited open files (-n) 65536 pipe size (512 bytes, -p) 8 POSIX message queues (bytes, -q) 819200 real-time priority (-r) 0 stack size (kbytes, -s) 32768 cpu time (seconds, -t) unlimited max user processes (-u) 16384 virtual memory (kbytes, -v) unlimited file locks (-x) unlimited soft limits according to ulimit: core file size (blocks, -c) unlimited data seg size (kbytes, -d) unlimited scheduling priority (-e) 0 file size (blocks, -f) unlimited pending signals (-i) 30554 max locked memory (kbytes, -l) 134217728 max memory size (kbytes, -m) unlimited open files (-n) 1024 pipe size (512 bytes, -p) 8 POSIX message queues (bytes, -q) 819200 real-time priority (-r) 0 stack size (kbytes, -s) 10240 cpu time (seconds, -t) unlimited max user processes (-u) 16384 virtual memory (kbytes, -v) unlimited file locks (-x) unlimited actual limits as per /proc: Limit Soft Limit Hard Limit Units Max cpu time unlimited unlimited seconds Max file size unlimited unlimited bytes Max data size unlimited unlimited bytes Max stack size 10485760 33554432 bytes Max core file size unlimited unlimited bytes Max resident set unlimited unlimited bytes Max processes 16384 16384 processes Max open files 1024 65536 files Max locked memory 137438953472 137438953472 bytes Max address space unlimited unlimited bytes Max file locks unlimited unlimited locks Max pending signals 30554 30554 signals Max msgqueue size 819200 819200 bytes Max nice priority 0 0 Max realtime priority 0 0 Max realtime timeout unlimited unlimited us -----------------------------------------------------------
As you can see, there are quite some differences making it necessary to set limits via systemd-specific syntax in the unit file itself. I can only conclude that systemd – as documented – does not pay attention to the configuration set by pam_limits(8).
OK, so shell limits aren’t respected, what next?
So if systemd doesn’t make use of pam_limits(8) there needs to be a different solution. There is a whole raft of options documented in systemd.directives(7) for Oracle Linux 8. Again, this might be different for Oracle Linux 7. The database needs the following shell limits to be set:
- open file descriptors (“nofile”)
- number of processes available to a single user (“nproc”)
- Size of the stack segment per process (“stack”)
- Maximum locked memory limit (“memlock”)
These map to the following systemd.directives(7):
LimitNOFILELimitNPROCLimitSTACKLimitMEMLOCK
You should really go ahead and read the man pages for systemd, they are great!
Amending the unit file
The next step is to add the above directives to the unit file, reload systemd, start the service and see what happens. Note that most sources I found only set LimitMEMLOCK and LimitNOFILE for Oracle. I am using the values from the Oracle documentation – your system might require different settings.
[Unit] Description=A oneshot service to test whether systemd respects PAM limits [Service] LimitNOFILE=1024:65536 LimitNPROC=2047:16384 LimitSTACK=10485760:33554432 LimitMEMLOCK=infinity User=oracle Group=oinstall Type=oneshot ExecStart="/home/oracle/test.sh"
Apart from the additional Limit.* directives, it’s the same file. The syntax in Oracle Linux 8 is LimitDIRECTIVE=soft:hard limit.
With the unit file changed, the following limits have been recorded:
Jun 16 20:30:57 server1 systemd[1]: Starting A oneshot service to test whether systemd respects PAM limits... Jun 16 20:30:57 server1 test.sh[13821]: ----------------------------------------------------------- Jun 16 20:30:57 server1 test.sh[13821]: test run starts at Wed Jun 16 20:30:57 UTC 2021: Jun 16 20:30:57 server1 test.sh[13821]: I am oracle Jun 16 20:30:57 server1 test.sh[13821]: my own PID is: 13821 Jun 16 20:30:57 server1 test.sh[13821]: oracle 13821 1 0 20:30 ? 00:00:00 bash /home/oracle/test.sh Jun 16 20:30:57 server1 test.sh[13821]: oracle 13824 13821 0 20:30 ? 00:00:00 /usr/bin/ps -ef Jun 16 20:30:57 server1 test.sh[13821]: hard limits according to ulimit: Jun 16 20:30:57 server1 test.sh[13821]: core file size (blocks, -c) unlimited Jun 16 20:30:57 server1 test.sh[13821]: data seg size (kbytes, -d) unlimited Jun 16 20:30:57 server1 test.sh[13821]: scheduling priority (-e) 0 Jun 16 20:30:57 server1 test.sh[13821]: file size (blocks, -f) unlimited Jun 16 20:30:57 server1 test.sh[13821]: pending signals (-i) 30554 Jun 16 20:30:57 server1 test.sh[13821]: max locked memory (kbytes, -l) unlimited Jun 16 20:30:57 server1 test.sh[13821]: max memory size (kbytes, -m) unlimited Jun 16 20:30:57 server1 test.sh[13821]: open files (-n) 65536 Jun 16 20:30:57 server1 test.sh[13821]: pipe size (512 bytes, -p) 8 Jun 16 20:30:57 server1 test.sh[13821]: POSIX message queues (bytes, -q) 819200 Jun 16 20:30:57 server1 test.sh[13821]: real-time priority (-r) 0 Jun 16 20:30:57 server1 test.sh[13821]: stack size (kbytes, -s) 32768 Jun 16 20:30:57 server1 test.sh[13821]: cpu time (seconds, -t) unlimited Jun 16 20:30:57 server1 test.sh[13821]: max user processes (-u) 16384 Jun 16 20:30:57 server1 test.sh[13821]: virtual memory (kbytes, -v) unlimited Jun 16 20:30:57 server1 test.sh[13821]: file locks (-x) unlimited Jun 16 20:30:57 server1 test.sh[13821]: soft limits according to ulimit: Jun 16 20:30:57 server1 test.sh[13821]: core file size (blocks, -c) unlimited Jun 16 20:30:57 server1 test.sh[13821]: data seg size (kbytes, -d) unlimited Jun 16 20:30:57 server1 test.sh[13821]: scheduling priority (-e) 0 Jun 16 20:30:57 server1 test.sh[13821]: file size (blocks, -f) unlimited Jun 16 20:30:57 server1 test.sh[13821]: pending signals (-i) 30554 Jun 16 20:30:57 server1 test.sh[13821]: max locked memory (kbytes, -l) unlimited Jun 16 20:30:57 server1 test.sh[13821]: max memory size (kbytes, -m) unlimited Jun 16 20:30:57 server1 test.sh[13821]: open files (-n) 1024 Jun 16 20:30:57 server1 test.sh[13821]: pipe size (512 bytes, -p) 8 Jun 16 20:30:57 server1 test.sh[13821]: POSIX message queues (bytes, -q) 819200 Jun 16 20:30:57 server1 test.sh[13821]: real-time priority (-r) 0 Jun 16 20:30:57 server1 test.sh[13821]: stack size (kbytes, -s) 10240 Jun 16 20:30:57 server1 test.sh[13821]: cpu time (seconds, -t) unlimited Jun 16 20:30:57 server1 test.sh[13821]: max user processes (-u) 2047 Jun 16 20:30:57 server1 test.sh[13821]: virtual memory (kbytes, -v) unlimited Jun 16 20:30:57 server1 test.sh[13821]: file locks (-x) unlimited Jun 16 20:30:57 server1 test.sh[13821]: actual limits as per /proc: Jun 16 20:30:57 server1 test.sh[13821]: Limit Soft Limit Hard Limit Units Jun 16 20:30:57 server1 test.sh[13821]: Max cpu time unlimited unlimited seconds Jun 16 20:30:57 server1 test.sh[13821]: Max file size unlimited unlimited bytes Jun 16 20:30:57 server1 test.sh[13821]: Max data size unlimited unlimited bytes Jun 16 20:30:57 server1 test.sh[13821]: Max stack size 10485760 33554432 bytes Jun 16 20:30:57 server1 test.sh[13821]: Max core file size unlimited unlimited bytes Jun 16 20:30:57 server1 test.sh[13821]: Max resident set unlimited unlimited bytes Jun 16 20:30:57 server1 test.sh[13821]: Max processes 2047 16384 processes Jun 16 20:30:57 server1 test.sh[13821]: Max open files 1024 65536 files Jun 16 20:30:57 server1 test.sh[13821]: Max locked memory unlimited unlimited bytes Jun 16 20:30:57 server1 test.sh[13821]: Max address space unlimited unlimited bytes Jun 16 20:30:57 server1 test.sh[13821]: Max file locks unlimited unlimited locks Jun 16 20:30:57 server1 test.sh[13821]: Max pending signals 30554 30554 signals Jun 16 20:30:57 server1 test.sh[13821]: Max msgqueue size 819200 819200 bytes Jun 16 20:30:57 server1 test.sh[13821]: Max nice priority 0 0 Jun 16 20:30:57 server1 test.sh[13821]: Max realtime priority 0 0 Jun 16 20:30:57 server1 test.sh[13821]: Max realtime timeout unlimited unlimited us Jun 16 20:30:57 server1 systemd[1]: limitstest.service: Succeeded. Jun 16 20:30:57 server1 test.sh[13821]: ----------------------------------------------------------- Jun 16 20:30:57 server1 systemd[1]: Started A oneshot service to test whether systemd respects PAM limits.
Thankfully this works, and the settings as documented by Oracle are implemented.
Summary
I always found it hard to come up with a working systemd unit file to start a single instance Oracle database. There is a wide range of articles covering SysV init, upstart, and systemd. It certainly can be confusing for a DBA to pick the right one. I have never been a great fan of using /etc/init.d for startup scripts in modern Linux distributions, even though Oracle Linux has a compatibility wrapper to deal with legacy startup scripts. After spending time with systemd I’m now comfortable writing my own start/stop script for RDBMS and listener. Maybe I’ll publish the results in a different post.
I hope you found the article useful for writing your own systemd unit files in Oracle Linux 7 and 8.
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