Re: increasing the default WAL segment size

Hi,

I was looking to commit this, but the changes I made ended up being
pretty large. Here's what I changed in the attached:
- split GUC_UNIT_BYTE into a separate commit, squashed rest
- renamed GUC_UNIT_BYT to GUC_UNIT_BYTE, don't see why we'd have such a
  weird abbreviation?
- bumped control file version, otherwise things wouldn't work correctly
- wal_segment_size text still said "Shows the number of pages per write
  ahead log segment."
- I still feel strongly that exporting XLogSegSize, which previously was
  a macro and now a integer variable, is a bad idea. Hence I've renamed
  it to wal_segment_size.
- There still were comments referencing XLOG_SEG_SIZE
- IsPowerOf2 regarded 0 as a valid power of two
- ConvertToXSegs() depended on a variable not passed as arg, bad idea.
- As previously mentioned, I don't think it's ok to rely on vars like
  XLogSegSize to be defined both in backend and frontend code.
- I don't think XLogReader can rely on XLogSegSize, needs to be
  parametrized.
- pg_rewind exported another copy of extern int XLogSegSize
- streamutil.h had a extern uint32 WalSegsz; but used
  RetrieveXlogSegSize, that seems needlessly different
- moved wal_segment_size (aka XLogSegSize) to xlog.h
- pg_standby included xlogreader, not sure why?
- MaxSegmentsPerLogFile still had a conflicting naming scheme
- you'd included "sys/stat.h", that's not really appropriate for system
  headers, should be <sys/stat.h> (and then grouped w/ rest)
- pg_controldata's warning about an invalid segsize missed newlines

Unresolved:
- this needs some new performance tests, the number of added instructions
  isn't trivial. Don't think there's anything, but ...
- read through it again, check long lines
- pg_standby's RetrieveWALSegSize() does too much for it's name. It
  seems quite weird that a function named that way has the section below
  "/* check if clean up is necessary */"
- the way you redid the ReadControlFile() invocation doesn't quite seem
  right. Consider what happens if XLOGbuffers isn't -1 - then we
  wouldn't read the control file, but you unconditionally copy it in
  XLOGShmemInit(). I think we instead should introduce something like
  XLOGPreShmemInit() that reads the control file unless in bootstrap
  mode. Then get rid of the second ReadControlFile() already present.
- In pg_resetwal.c:ReadControlFile() we ignore the file contents if
  there's an invalid segment size, but accept the contents as guessed if
  there's a crc failure - that seems a bit weird?
- verify EXEC_BACKEND does the right thing
- not this commit/patch, but XLogReadDetermineTimeline() could really
  use some simplifying of repetitive expresssions
- XLOGShmemInit shouldn't memcpy to temp_cfile and such, why not just
  save previous pointer in a local variable?
- could you fill in the Reviewed-By: line in the commit message?

Running out of concentration / time now.

- Andres