Erik Jones wrote:
On Apr 5, 2007, at 12:09 PM, Xiaoning Ding wrote:
Hi,
A page may be double buffered in PG's buffer pool and in OS's buffer
cache.
Other DBMS like DB2 and Oracle has provided Direct I/O option to eliminate
double buffering. I noticed there were discusses on the list. But
I can not find similar option in PG. Does PG support direct I/O now?
The tuning guide of PG usually recommends a small shared buffer pool
(compared
to the size of physical memory). I think it is to avoid swapping. If
there were
swapping, OS kernel may swap out some pages in PG's buffer pool even PG
want to keep them in memory. i.e. PG would loose full control over
buffer pool.
A large buffer pool is not good because it may
1. cause more pages double buffered, and thus decrease the efficiency of
buffer
cache and buffer pool.
2. may cause swapping.
Am I right?
If PG's buffer pool is small compared with physical memory, can I say
that the
hit ratio of PG's buffer pool is not so meaningful because most misses
can be
satisfied by OS Kernel's buffer cache?
Thanks!
To the best of my knowledge, Postgres itself does not have a direct IO
option (although it would be a good addition). So, in order to use
direct IO with postgres you'll need to consult your filesystem docs for
how to set the forcedirectio mount option. I believe it can be set
dynamically, but if you want it to be permanent you'll to add it to your
fstab/vfstab file.
I use Linux. It supports direct I/O on a per-file basis only. To
bypass OS buffer cache,
files should be opened with O_DIRECT option. I afraid that I have to
modify PG.
Xiaoning
erik jones <erik@xxxxxxxxxx <mailto:erik@xxxxxxxxxx>>
software developer
615-296-0838
emma(r)
