One of the work areas happening for Postgres 10 currently in development is the problem related to data durability when using the in-core utilities to take data dumps, base backups, or anything related to Postgres like WAL segments. In short, the goal is for users to be sure that any data taken from Postgres using the in-core tools is still present and consistent on disk even if a power failure occurs once the binary writing and flushing the data is done. A couple of days ago the following commit has landed in the code tree to improve the situation for pg_basebackup and pg_receivexlog:
commit: bc34223bc1e2c51dff2007b3d3bd492a09b5a491 author: Peter Eisentraut <firstname.lastname@example.org> date: Thu, 29 Sep 2016 12:00:00 -0400 pg_basebackup pg_receivexlog: Issue fsync more carefully Several places weren't careful about fsyncing in the way. See 1d4a0ab1 and 606e0f98 for details about required fsyncs. This adds a couple of functions in src/common/ that have an equivalent in the backend: durable_rename(), fsync_parent_path() From: Michael Paquier
It may not sound that complicated, but the devil is in the details here. On top of a refactoring to make some routines already used by initdb available as well for other binaries, a close lookup has been necessary to determine when data can be flushed to disk safely and how it should happen. Of course, cases where for example a base backup is returned directly to stdout, there is no way for pg_basebackup to guarantee that the data will still be here, so it is up to the caller to make sure that data is consistently on disk.
For example, one corner case is at the creation of a WAL segment which is padded of 16MB of zeros at creation in a code path used by both pg_basebackup or pg_receivexlog. In the event of a crash between the zero-padding and the fsync() of the WAL segment created, it may be possible that the segment is still here with a size of 16MB, in which case it is safer to issue an additional fsync() when opening it.
Note that by default pg_basebackup will flush the data when it thinks it is necessary to do so, for both the tar and normal format. For people caring more about about performance that data consistency, a new –nosync option has been added to pg_basebackup with this commit. to emulate the pre-10 behavior. Though it is very encouraged to not use it except for test environments if you care about your data. This is not present for pg_receivexlog because in its case reliability is the priority, and that would be quite confusing with the existing –synchronous option anyway.
Future improvements are also aimed for pg_dump, likely a patch will be made for the next commit fest beginning in November for integration in Postgres 10. As the infrastructure is in place, this should require less efforts.