On Apr 5, 2007, at 1:27 PM, Mark Lewis wrote:
On Thu, 2007-04-05 at 13:09 -0500, 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.
Not to hijack this thread, but has anybody here tested the behavior of
PG on a file system with OS-level caching disabled via forcedirectio or
by using an inherently non-caching file system such as ocfs2?
I've been thinking about trying this setup to avoid double-caching now
that the 8.x series scales shared buffers better, but I figured I'd ask
first if anybody here had experience with similar configurations.
-- Mark
Rather than repeat everything that was said just last week, I'll point out that we just had a pretty decent discusson on this last week that I started, so check the archives. In summary though, if you have a high io transaction load with a db where the average size of your "working set" of data doesn't fit in memory with room to spare, then direct io can be a huge plus, otherwise you probably won't see much of a difference. I have yet to hear of anybody actually seeing any degradation in the db performance from it. In addition, while it doesn't bother me, I'd watch the top posting as some people get pretty religious about (I moved your comments down).
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