Recapitulating and addressing some of the issues with my previous
attempt at this feature:
PostgreSQL currently uses the System V shared memory APIs to access
shared memory. On Mac OS X and other BSDs,
the default System V shared memory limits are often very low and
require adjustment for acceptable performance. Par-
ticularly, when Postgres is included as part of larger end-user
friendly software products, these kernel settings are often
difficult to change programatically.
The System V shared memory facilities provide a method to determine
who is attached to a shared memory segment.
This is used to prevent backends that were orphaned by crashed or
killed database processes from corrupting the data-
base as it is restarted. The same effect can be achieved with using
the POSIX APIs, but since the POSIX library does not
have a way to check who is attached to a segment, atomic segment
creation must be used to ensure exclusive access to
the database.
In order for this to work, the key name used to open and create the
shared memory segment must be unique for each
data directory. This is done by using a strong hash of the canonical
form of the data directory’s pathname. This also re-
moves any risk of other applications, or other databases’ memory
segments colliding with the current shared memory
segment, which conveniently simplifies the logic.
The algorithm changes are described by the before and after flowchart
PDF included in the tarball below.
There is also a Windows version of this patch included, which can
replace the current SysV-to-Win32 shared memory
layer as it currently does not check for orphaned backends in the
database. If this is used,
src/backend/port/win32/shmem.c and its makefile reference can be
removed.
Included is a patch for the configure.in file to opt-in all of the
supported platforms to use this POSIX shared memory
layer except those that are explicitly excluded (of course, this
could be done in reverse if desired). Notably, NetBSD and
OpenBSD do not support the POSIX shared memory calls and their
template patches are included. Other platforms
should be tested and excluded as needed. In addition, the
documentation section 16.4.1 ‘Shared Memory and Sema-
phores’ could be largely pruned and updated to reflect use of the
POSIX calls.
The patches are available here (the postings were being silently
dropped when I attached this large of a file):
http://homepage.mac.com/cmarcellino/postgres-posix-shmem.tar
Please let me know if there is a better way to post this.
Thanks for your feedback,
Chris Marcellino
