Nothing is guaranteed for anything larger than 512 bytes, and even
then you have maybe 1e-13 likelihood of a badly-written block written
during a power outage going unnoticed. (That is why the FAQ recommends
you invest in a UPS.) If PG crashes, you're covered, regardless of
block size. If the OS crashes, you're not. If the power goes out,
you're not.
The block size affects how much is written when you change only a
single record within a block. When you update a two-byte field in a
100-byte record, do you want to write 32k? (The answer is "maybe".)
Nathan Myers
ncm@zembu.com
On Tue, Nov 28, 2000 at 09:14:15AM +0800, Christopher Kings-Lynne wrote:
> I don't believe it's a performance issue, I believe it's that writes to
> blocks greater than 8k cannot be guaranteed 'atomic' by the operating
> system. Hence, 32k blocks would break the transactions system. (Or
> something like that - am I correct?)
>
> > From: pgsql-hackers-owner@postgresql.org <On Behalf Of Mitch Vincent>
> > Sent: Tuesday, November 28, 2000 8:40 AM
> > Subject: Re: [HACKERS] 8192 BLCKSZ ?
> >
> > I've been using a 32k BLCKSZ for months now without any trouble,
> > though I've
> > not benchmarked it to see if it's any faster than one with a
> > BLCKSZ of 8k..
> >
> > > This is just a curiosity.
> > >
> > > Why is the default postgres block size 8192? These days, with caching
> > > file systems, high speed DMA disks, hundreds of megabytes of RAM, maybe
> > > even gigabytes. Surely, 8K is inefficient.
> > >
> > > Has anyone done any tests to see if a default 32K block would provide a
> > > better overall performance? 8K seems so small, and 32K looks to be where
> > > most x86 operating systems seem to have a sweet spot.
> > >
> > > If someone has the answer off the top of their head, and I'm just being
> > > stupid, let me have it. However, I have needed to up the block size to
> > > 32K for a text management system and have seen no performance problems.
> > > (It has not been a scientific experiment, admittedly.)
> > >
> > > This isn't a rant, but my gut tells me that a 32k block size as default
> > > would be better, and that smaller deployments should adjust down as
> > > needed.
