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@xxxxxxxxxxxxxxx Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
