According to the official PostgreSQL documentation:
Sets the amount of memory the database server uses for shared memory buffers. The default is typically 128 megabytes (128MB), but might be less if your kernel settings will not support it (as determined during initdb). This setting must be at least 128 kilobytes. (Non-default values of BLCKSZ change the minimum.) However, settings significantly higher than the minimum are usually needed for good performance. This parameter can only be set at server start.
Is anyone aware of any way of increasing this parameter without restarting the process or (if you have to) without affecting an app that is making a bunch of batched UPDATE/DELETEs on the database?