On 04/30/2011 12:24 AM, Hsien-Wen Chu wrote:
> I'm little bit confuse why it is not safe. and my question is following.
>
> for database application, we need to avoid double cache, PostgreSQL
> shared_buffer will cache the data, so we do not want to file system to
> cache the data right?. so the DIRECT IO is better, right?.
>
No. There are parts of PostgreSQL that expect the operating system to
do write caching. Two examples are the transaction logs and the
processing done by VACUUM. If you eliminate that with direct I/O, the
slowdown can be much, much larger than what you gain by eliminating
double-buffering on reads.
On the read side, PostgreSQL also expects that operating system features
like read-ahead are working properly. While this does introduce some
double-buffering, the benefits for sequential scans are larger than that
overhead, too. You may not get the expected read-ahead behavior if you
use direct I/O.
Direct I/O is not a magic switch that makes things faster; you have to
very specifically write your application to work around what it does,
good and bad, before it is expected to improves things. And PostgreSQL
isn't written that way. It definitely requires OS caching to work well.
> for VXFS, if the we use ioctl(fd,vx_cacheset,vx_concurrent) API,
> according to the vxfs document, it will hold a shared lock for write
> operation, but not the exclusive clock, also it is a direct IO,
>
There are very specific technical requirements that you must follow when
using direct I/O. You don't get direct I/O without also following its
alignment needs. Read the "Direct I/O best practices" section of
http://people.redhat.com/msnitzer/docs/io-limits.txt for a quick intro
to the subject. And there's this additional set of requirements you
mention in order for this particular VXFS feature to work, which I can't
even comment on. But you can be sure PostgreSQL doesn't try to do
either of those things--it's definitely not aligning for direct I/O.
Has nothing to do with ACID or the filesystem.
Now, the VXFS implementation may do some tricks that bypass the
alignment requirements. But even if you got it to work, it would still
be slower for anything but some read-only workloads. Double buffering
is really not that big of a performance problem, you just need to make
sure you don't set shared_buffers to an extremely large value.
--
Greg Smith 2ndQuadrant US greg@2ndQuadrant.com Baltimore, MD
PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us
"PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books
