Re: Allowing WAL fsync to be done via O_SYNC

Tom Lane writes:

> wal_sync_method = fsync | fdatasync | open_sync | open_datasync

> A small problem is that I don't want to be doing multiple strcasecmp's
> to figure out what to do in xlog.c.

This should be efficient:

switch(lower(string[0]) + lower(string[5]))
{
case 'f': /* fsync */
case 'f' + 's': /* fdatasync */
case 'o' + 's': /* open_sync */
case 'o' + 'd': /* open_datasync */
}

Although ugly, it should serve as a readable solution for now.

> Do you object if I add an "assign_hook" to guc.c that's called when an
> actual assignment is made?

Something like this is on my wish list, but I'm not sure if it's wise to
start this now. There are a few issues that need some thought, like how
to make the interface for non-string options, and how to keep it in sync
with the parse hook of string options, ...

> That would provide a place to set up the flag variables that xlog.c
> would actually look at. Furthermore, having an assign_hook would let
> us support changing this value at SIGHUP, not only at postmaster
> start. (The assign hook would just need to fsync whatever WAL file is
> currently open and possibly close/reopen the file, to ensure that no
> blocks miss getting synced when we change conventions.)

... and possibly here you need to pass the context to the assign hook as
well. This application strikes me as a bit too esoteric for a first try.

--
Peter Eisentraut peter_e(at)gmx(dot)net http://yi.org/peter-e/