Thread

Re: [HACKERS] Tape files and MAXBLCKSZ vs. BLCKSZ

Bruce Momjian <maillist@candle.pha.pa.us> — 1998-01-07T02:19:16Z

> 
> > I can take a stab at this tonite after work now that the snapshot is there.
> > Still have around some of the files/diffs from looking at this a year ago...
> > 
> > I don't think it will be hard, just a few files with BLCKSZ/MAXBLCKSZ
> > references to check for breakage.  Appears that only one bit of lp_flags is
> > being used too, so that would seem to allow up to 32k blocks.
> 
> I have finished "fixing" the code for this and have a test system of postgres
> running with 4k blocks right now.  Tables appear to take about 10% less space.
> Simple btree indices are taking the same as with 8k blocks.  Regression is
> running now and is going smoothly.
> 
> Now for the question...
> 
> In backend/access/nbtree/nbtsort.c, ---> #define TAPEBLCKSZ (MAXBLCKSZ << 2)
> 
> So far MAXBLCKSZ has been equal to BLCKSZ.  What effect will a MAXBLCKSZ=32768
> have on these tape files?  Should I leave it as MAXBLCKSZ this big or change
> them to BLCKSZ to mirror the real block size being used?
> 

I would keep it equal to BLCKSZ.  I see no reason to make it different,
unless the btree sorting is expecting to take 2x the block size.  Vadim
may know.


> 
> > I can check the aix compiler, but what does gcc and other compilers do with
> > bit field alignment?
> 
> The ibm compiler allocates the ItemIdData as four bytes.  My C book says though
> that the individual compiler is free to align bit fields however it chooses.
> The bit-fields might not always be packed or allowed to cross integer boundaries.
> 
> darrenk
> 
> 


-- 
Bruce Momjian
maillist@candle.pha.pa.us