Setting sysctl values

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Setting sysctl values#

By default, TPA sets various sysctl values on target instances, and includes them in /etc/sysctl.conf so that they persist across reboots.

You can optionally specify your own values in sysctl_values :

cluster_vars:
  sysctl_values:
    kernel.core_pattern: core.%e.%p.%t
    vm.dirty_bytes: 4294967296
    vm.zone_reclaim_mode: 0

Any values you specify will take precedence over TPA’s default values for that variable (if any). The settings will first be added to sysctl.conf line-by-line, and finally loaded with sysctl -p .

Docker and lxd instances do not support setting sysctls, so TPA will skip this step altogether for those platforms.

Hugepages#

By default, TPA reserves hugepages of the architecture’s default size (2MB on x86_64) for Postgres to use for shared_buffers . It does this by adding hugepages=N to the kernel command line and writing vm.nr_hugepages = N to /etc/sysctl.conf .

If you want Postgres to use a larger hugepage size (typically 1GB on x86_64), set huge_page_size to a Postgres-style memory string:

instances:
  - Name: primary
    vars:
      huge_page_size: 1GB

When huge_page_size is set, TPA will:

  • reserve pages of that size on the kernel command line, e.g. hugepagesz=1G hugepages=N ;

  • omit vm.nr_hugepages from /etc/sysctl.conf (that sysctl only ever applies to the architecture’s default-size pool, and is silently ineffective for any other size); and

  • set the huge_page_size GUC in postgresql.conf so Postgres uses pages from the chosen pool.

It is almost always right to set huge_page_size on individual Postgres instances rather than in cluster_vars : any reservation made on the kernel command line is taken out of normal memory at boot, so a Barman server or other non-Postgres instance in the same cluster does not want the same allocation as a Postgres node.

By default, TPA calculates the number of hugepages to reserve as shared_buffers plus 3% of total RAM, divided by the hugepage size. You can override this calculation by setting nr_hugepages directly:

instances:
  - Name: primary
    vars:
      huge_page_size: 1GB
      nr_hugepages: 16

Note

For backwards compatibility, an existing sysctl_values['vm.nr_hugepages'] is honoured as the page count when nr_hugepages is not set explicitly.

By default, TPA will set transparent_hugepage to “never” on the kernel command line and also install a script, /etc/tpa/rc.local , to set it at boot-time via the sys filesystem. To use a different setting, set transparent_hugepage to always or madvise in an instances vars or in cluster_vars to control all instances.

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