You should rather consider VxFS tuning - it has an auto-discovery for DIRECT I/O according the block size. Just change this setting to 8K or 16-32K depending on your workload - then all I/O operations with a bigger block size will be executed in DIRECT mode and bypass FS cache (which logical as usually it'll correspond to a full scan or a seq scan of some data), while I/O requests with smaller blocks will remain cached which is very useful as it'll mainly cache random I/O (mainly index access).. With such a tuning I've got over %35 performance improvement comparing to any other states (full DIRECT or fully cached). Rgds, -Dimitri Rgds, -Dimitri On 5/5/11, Robert Haas <robertmhaas@xxxxxxxxx> wrote: > On Sat, Apr 30, 2011 at 4:51 AM, Hsien-Wen Chu <chu.hsien.wen@xxxxxxxxx> > 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 > > -- > Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) > To make changes to your subscription: > http://www.postgresql.org/mailpref/pgsql-performance > -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
