I wonder if kernel can sometimes provide weaker version of fsync() which
is not enforcing all pending data to be written immediately but just
servers as write barrier, guaranteeing
that all write operations preceding fsync() will be completed before any
of subsequent operations.
It will allow implementation of weaker transaction models which are not
satisfying all ACID requirements (results of committed transaction can
be lost in case power failure or OS crash) but still preserving database
consistency. It is acceptable for many applications and can provide much
better performance.
Right now it is possible to implement something like this at application
level using asynchronous write process. So all write/sync operations
should be redirected to this process.
But such process can become a bottleneck reducing scalability of the
system. Also communication channels with this process can cause
significant memory/CPU overhead.
In most DBMSes including PostgreSQL transaction log and database data
are located in separate files. So such write barrier should be
associated not with one file, but with set of files or may be the whole
file system. I wonder if there are some principle problems in
implementing or using such file system write barrier?