Re: PostgreSQL

On Wed, 1 Feb 2006, Alfred Perlstein wrote:

> Basically, FreeBSD has a setting 'kern.ipc.shm_use_phys' under sysctl
> that will tell the kernel not to make SysV shared memory pageable.  This
> is VERY handy when you are going to use very large shared segments.

Sure, but assuming your buffer cache works reasonably well, you ought
not be devoting more than about 20% of your memory to PostgreSQL shared
memory buffers, so they're hardly likely to get swapped anyway.

Keep in mind that those shared memory buffers are essentially
"double-buffered;" any block in a shared memory buffer is copied from
a block in the buffer cache, and it's used often enough, if it's in a
buffer, that it's likely to stay in the buffer cache as well. So by
increasing your shared memory buffers, you are in most circumstances
actually decreasing the amount of total cache you have available.

Some will argue that you should try and make the shared buffers huge
in order to minimize the amount of file system buffering, but because
you don't generally have much control over other memory usage (system
processes, other postgresql processes, sort memory, maintenance memory
for vacuum, etc.) you're highly likely to create truly tragic situations
such as your sort memory being swapped out to disk that will cause your
performance to plummet.

> You are very correct here.  MySQL is not a "real database", it's close,

I was using the term DBMS, or "Data Base Managment System"; a "database"
is what it manages. MySQL is not even close to a DBMS. But let's not get
into that. :-)

cjs
-- 
Curt Sampson            <[email protected]>             +81 90 7737 2974
   The power of accurate observation is commonly called cynicism
   by those who have not got it.    --George Bernard Shaw