Re: Recomended FS - Mailing list pgsql-general

On Tue, 21 Oct 2003, Ben-Nes Michael wrote:

> what about mirroring only ? raid 1 ?
>
> I always thought that raid 1 is the fastest, am I true ?
>
> I don't really need more then 3GB data and I have two 36GB HD. so I don't
> need lvl 0 nor lvl 5 unless raid 1 is slower.

    Raid 1 should not be slower than raid 5. hence

Raid 0
Write = Deciede which disk, Write
Read = Deciede Which disk, Read

Raid 1
Write = Write Disk 1, Write Dist 2
Read = Read (Don't matter which one)

Raid 5
Write = Write Disk 1, Write Disk 2, Calc Check Sum, Write Disk 3
Read = Read Disk 1, Read Disk 2, Regenate Data.

Peter Childs

>
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> ----- Original Message -----
> From: "Markus Wollny" <Markus.Wollny@computec.de>
> To: <holger@marzen.de>
> Cc: <pgsql-general@postgresql.org>
> Sent: Tuesday, October 21, 2003 11:00 AM
> Subject: Re: [GENERAL] Recomended FS
>
>
> > Theory vs. real life. In Theory, RAID5 is faster because less
> > data have
> > to be written to disk. But it's true, many RAID5 controllers
> > don't have
> > enough CPU power.
>
> I think it might not be just CPU-power of the controller. For RAID0+1
> you just have two disc-I/O per write-access: writing to the original set
> and the mirror-set. For RAID5 you have three additional
> disc-I/O-processes: 1. Read the original data block, 2. read the parity
> block (and calculate the new parity block, which is not a disk I/O), 3.
> write the updated data block and 4. write the updated parity block. Thus
> recommendations by IBM for DB/2 and several Oracle-consultants state
> that RAID5 is the best compromise for storage vs. transaction speed, but
> if your main concern is the latter, you're always best of with RAID0+1;
> RAID0+1 does indeed always and reproducably have better write
> performance that RAID0+1 and read-performance is almost always also
> slightly better.
>
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