On Sat, Apr 30, 2011 at 4:51 AM, Hsien-Wen Chu <chu.hsien.wen@gmail.com> wrote:
> since the block size is 8k for the default, and it consisted with many
> tuple/line; as my understand, if any tuple/line is changed(maybe
> update, insert, delete). the block will be marked as dirty block. and
> then it will be flashed to disk by bgwriter.
True...
> so my question is if the data block(8k) is aligned with the file
> system block? if it is aligned with file system block, so what's the
> potential issue make it is not safe for direct io. (please assume
> vxfs, vxvm and the disk sector is aligned ).please correct me if any
> incorrect.
It's not about safety - it's about performance. On a machine with
64GB of RAM, a typical setting for shared_buffers set to 8GB. If you
start reading blocks into the PostgreSQL cache - or writing them out
of the cache - in a way that bypasses the filesystem cache, you're
going to have only 8GB of cache, instead of some much larger amount.
More cache = better performance.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company