From PostgreSQL wiki
'Group commit is a feature planned for PostgreSQL 9.2 . The feature is being developed by Simon Riggs and Peter Geoghegan. The latest -hackers thread on the feature is: http://archives.postgresql.org/pgsql-hackers/2012-01/msg00804.php .
Description of feature
Broadly speaking, a group commit feature enables PostgreSQL to commit a group of transactions in batch, amortizing the cost of flushing WAL. The proposed implementation this page describes is heavily based on the existing synchronous replication implementation.
Benchmarking of this feature has been performed by Greg Smith's pgbench tool (https://github.com/gregs1104/pgbench-tools) . Here are results for the initial benchmark:
These results were obtained on an ext4 (Linux kernel 3.1) filesystem with LVM. The harddisk used was a WDC WD3200BEKT-08PVMT1 7200 RPM sata disk, with write caching enabled.