VX_CONCURRENT flag on vxfs( 5.1 or later) for performance for postgresql?

Dear all

When database files are on a VxFS filesystem, performance can be
significantly improved by setting the VX_CONCURRENT cache advisory on
the file according to vxfs document,

my question is that have any tested by this?

#include <sys/fs/vx_ioctl.h>
ioctl(fd, VX_SETCACHE, VX_CONCURRENT);

Regards

HSIEN WEN

HSIEN-WEN CHU <hsienwen(dot)chu(at)googlemail(dot)com> writes:
> When database files are on a VxFS filesystem, performance can be
> significantly improved by setting the VX_CONCURRENT cache advisory on
> the file according to vxfs document,

Presumably, if whatever behavior this invokes were an unalloyed good,
they'd have just made it the default. The existence of a flag makes
me suppose that there are some clear application-visible downsides.
What are they?

BTW, please do not cross-post the same question to three different lists.

regards, tom lane

On 04/27/2011 11:33 PM, HSIEN-WEN CHU wrote:
> When database files are on a VxFS filesystem, performance can be
> significantly improved by setting the VX_CONCURRENT cache advisory on
> the file according to vxfs document,
>

That won't improve performance, and it's not safe either. VX_CONCURRENT
switches the filesystem to use direct I/O. That's usually slower for
PostgreSQL. And it introduces some requirements for both block
alignment and the application avoiding overlapping writes. PostgreSQL
doesn't do either, so I wouldn't expect it to be compatible with
VX_CONCURRENT.

--
Greg Smith 2ndQuadrant US greg(at)2ndQuadrant(dot)com Baltimore, MD
PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us