Re: fallocate / posix_fallocate for new WAL file creation (etc...)

On Thu, May 30, 2013 at 02:58:26PM +0200, Andres Freund wrote:
> > * Andres Freund (andres(at)2ndquadrant(dot)com) wrote:
> > > But really, I am not at all concerned about some obscure values being
> > > returned, but about a read() not being successful..

> After a bit of standard perusing writing a single byte to the end of the
> file after the fallocate ought to make at least the reading guaranteed
> to be defined. If we did seek(last_byte); write(); posix_fallocate() we
> should even always have defined content. Yuck.

This portion of the posix_fallocate() specification requires the hoped-for
effect on subsequent read() calls:

If the offset+ len is beyond the current file size, then posix_fallocate()
shall adjust the file size to offset+ len. Otherwise, the file size shall
not be changed.
-- http://pubs.opengroup.org/onlinepubs/9699919799/functions/posix_fallocate.html

When the file size increases, read()'s defined behavior switches from
returning short to retrieving zeros. There's no need for an additional
write() to ensure that.

--
Noah Misch
EnterpriseDB http://www.enterprisedb.com