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Re: Shared buffers, db transactions commited, and write IO on Solaris

From: Dimitri <dimitrik(dot)fr(at)gmail(dot)com>
To: "Erik Jones" <erik(at)myemma(dot)com>
Cc: "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us>, "PostgreSQL Performance" <pgsql-performance(at)postgresql(dot)org>
Subject: Re: Shared buffers, db transactions commited, and write IO on Solaris
Date: 2007-03-30 13:14:35
Message-ID: 5482c80a0703300614x1fe7d9ccy2d414ed87f98db3d@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-performance
>
> You are right in that the page size constraint is lifted in that
> directio cuts out the VM filesystem cache.  However, the Solaris
> kernel still issues io ops in terms of its logical block size (which
> we have at the default 8K).  It can issue io ops for fragments as
> small as 1/8th of the block size, but Postgres issues its io requests
> in terms of the block size which means that io ops from Postgres will
> be in 8K chunks which is exactly what we see when we look at our
> system io stats.  In fact, if any io request is made that isn't a
> multiple of 512 bytes (the disk sector size), the file system
> switches back to the buffered io.

Oh, yes, of course! yes, you still need to respect multiple of 512
bytes block size on read and write - sorry, I was tired :)

Then it's seems to be true - default XLOG block size is 8K, means for
every even small auto-committed transaction we should write 8K?... Is
there any reason to use so big default block size?...

Probably it may be a good idea to put it as 'initdb' parameter? and
have such value per database server?

Rgds,
-Dimitri

>
> >
> > However, to understand TX number mystery I think the only possible
> > solution
> > is to reproduce a small live test:
> >
> > (I'm sure you're aware you can mount/unmount forcedirectio
> > dynamically?)
> >
> > during stable workload do:
> >
> >   # mount -o remount,logging  /path_to_your_filesystem
> >
> > and check if I/O volume is increasing as well TX numbers
> > than come back:
> >
> >   # mount -o remount,forcedirectio  /path_to_your_filesystem
> >
> > and see if I/O volume is decreasing as well TX numbers...
>
> That's an excellent idea and I'll run it by the rest of our team
> tomorrow.
>
> erik jones <erik(at)myemma(dot)com>
> software developer
> 615-296-0838
> emma(r)
>
>
>
>

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