Re: Files or RAW-Devices ?
Date: 1996/04/03
Message-ID: <3162E773.4101_at_teldta.com>#1/1
Wachs Rene wrote:
>
> Hello
>
> This summer our DEC Alpha 4000 will be replaced by a DEC 8xxxx. Today we have a 20
> GB-DB with mainly 2 large-tabs (6.5 GB-Tab with 12 mio. rows, 3.5 GB-Tab with 7
> mio. rows). Performance is satisfactory, but sometimes multiuser behaviour is
> really bad. Nobody can explain that - neither Digital nor Oracle.
>
> DEC says : on the new DEC-Box don't make Files ! Make raw-Devices ! This will
> increase performance and improve the multiuser behaviour.
>
> My questions :
>
> - is this true ? Advantages of raw-devices against files ?
> - which conditions have to be done for raw-devices ?
> - raw-devices pitfalls ?
> - practical experiences ?
>
> Thanks very much.
>
> R. Wachs
- is this true ?
Yes. In all tuning books I have read including Oracle's it suggests using raw devices. I have also done some timings on a Sun Sparc 1000 with Solaris 2.3 using a Sparc Array via Oracle 7.1.3. I mixed and matched configurations to include different levels of raid and raw/UFS. In short, the best was raid 0+1 (striping+mirroring).
- Advantages of raw-devices against files ?
No more overhead of the system cache (remember to lower the UNIX system cache value, a large one is not necessary anymore). I/O is direct and therefor faster. The work that UNIX must do to maintain the file system (UFS) is gone. Most UNIX file systems like to reserve 10% of any space for overhead, fragmentation, i-nodes, etc. (or something like that) leaving only 90% usable. With raw devices the entire partition, all 100% is available for Oracle tablespace datafiles.
- which conditions have to be done for raw-devices ?
I'm not sure what you mean hear. How about this, all parts of the db can be raw including the redo logs (I have not tried this but it's supposed to be do-able) excluding the archive area.
- raw-devices pitfalls ?
must use the UNIX 'dd' command or something at the device level to do backups. No more cp/compress/tar/etc. You must brown nose the system administrator so they will do the disk partition for you.
- practical experiences ?
I have had positive experiences with raw devices for the last 5 years on 5-10g databases. Hopefully you will find this helpful.
brian.maclean_at_teldta.com
(not responsible for lost or stolen data)
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