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Re: Max IO size

From: Connor McDonald <connor_mcdonald_at_yahoo.com>
Date: Fri, 07 Sep 2001 19:05:28 +0100
Message-ID: <3B990C68.19F7@yahoo.com>


Connor McDonald wrote:
>
> Svend Jensen wrote:
> >
> > Hi
> >
> > The underlying hardware has not unlimited read capasity, and most systems
> > and disk / controler's have the limit at 64k reads on one IO. Many a time
> > the
> > hardware ventors/sales/tech people dont know what the specific hardware can
> > deliver.
> > Solaris 7 & 6 unix fsys had a io-size of 8k and a default max read size of
> > 7 pages.
> > Saying the operation system read 56k in one disk IO. Not what most people
> > expect.
> > This could be changed to some degree. (I'm not Solaris expert)
> >
> > On Solaris 7 Oracle 8.1.6 I tried to set db_file_multiblock_read_count to
> > 16384,
> > and ran a full scan on a huge table (1600Mb) and tracing level 12. The
> > trace file
> > waits gave that the server process never asked for more than 127 blocks
> > (8k).
> >
> > Different disk subsystems on one system - might behave different,
> > and to stop guesswork - just test the system. Start the trace and see what
> > the database engine demands and some other tool to see what the OS system
> > delivers. And maybe compare numbers from v$filestat and iostat (?) output.
> > If v$filestat numbers are half the IO's from iostat, the you know that the
> > OS
> > only delivers half of the read size Oracle demands. One Oracle pio is the
> > converted to two physical io's by the operating system.
> >
> > Anyway - in real life (active) systems, the server process might not ask
> > for
> > very many continous reads from disk, because some blocks from the target
> > will be in buffer cache, and they are not requested re-read! Hence the
> > request
> > will/might anyway be devided into several pio's, like read 7 blocks (next
> > two
> > are in cache), read 12 blocks (next++ is there), read 2...read 8..read
> > 5....
> > The higher the demand is for an object - the smaller the average pio,
> > because
> > more blocks will sit in the cache already.
> >
> > /Svend Jensen
> >
> > Connor McDonald wrote:
> >
> > > I've got a sun (solaris 2.8) box with with datafiles under vxfs. After
> > > setting
> > >
> > > - maxphys to 512k
> > > - vxiomax to 512k
> > > - db block size to 8k
> > > - multiblock read count to 64
> > >
> > > I had presumed that I could get a ceiling of half a meg per read on
> > > (say) a full table scan, but a level 8 10046 trace yields a ceiling of
> > > 256k (ie p3 in the waits is 32 not 64)
> > >
> > > I'll be repeating the test under a raw partition tomorrow but comments
> > > anyone ? As a related question, anyone know what Oracle's SSTIOMAX is
> > > under the various versions (say from 7.3)
> > >
> > > Cheers
> > >
> > > --
> > > ==============================
> > > Connor McDonald
> > >
> > > http://www.oracledba.co.uk
> > >
> > > "Some days you're the pigeon, some days you're the statue..."
>
> Appears to be a file system restriction. The same test on a *raw* vxvm
> volume can get up to 1m (with maxphys and vxiomax at 1m,
> multiblock-read-count at 128 on block size 8k). But smack a file system
> on top of it and ker-bang, I'm back down to 256k...
>
> Yet *another* reason why I like raw and hate file systems !
>
> Cheers
> Connor
> --
> ==============================
> Connor McDonald
>
> http://www.oracledba.co.uk
>
> "Some days you're the pigeon, some days you're the statue..."

Resolved with some additional settings when creating the file systems (maxcontig et al)

Still - gimme raw anyday!

Cheers
Connor

-- 
==============================
Connor McDonald

http://www.oracledba.co.uk

"Some days you're the pigeon, some days you're the statue..."
Received on Fri Sep 07 2001 - 13:05:28 CDT

Original text of this message

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