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Re: 8192 BLCKSZ ?

From: Nathan Myers <ncm(at)zembu(dot)com>
To: pgsql-hackers(at)postgresql(dot)org
Subject: Re: 8192 BLCKSZ ?
Date: 2000-11-28 01:49:46
Message-ID: 20001127174946.A22345@store.zembu.com (view raw or flat)
Thread:
Lists: pgsql-hackers
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(at)zembu(dot)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(at)postgresql(dot)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.

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