Bruce Momjian wrote: > Greg Smith wrote: >> A good test program that is a bit better at introducing and detecting >> the write cache issue is described at >> http://brad.livejournal.com/2116715.html > > Wow, I had not seen that tool before. I have added a link to it from > our documentation, and also added a mention of our src/tools/fsync test > tool to our docs.
One challenge with many of these test programs is that some filesystem (ext3 is one) will flush drive caches on fsync() *sometimes, but not always. If your test program happens to do a sequence of commands that makes an fsync() actually flush a disk's caches, it might mislead you if your actual application has a different series of system calls. For example, ext3 fsync() will issue write barrier commands if the inode was modified; but not if the inode wasn't. See test program here: http://www.mail-archive.com/linux-ker...@vger.kernel.org/msg272253.html and read two paragraphs further to see how touching the inode makes ext3 fsync behave differently. -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
