Re: [ADMIN] H/W RAID 5 on slower disks versus no raid on

On Thu, 21 Nov 2002, Bjoern Metzdorf wrote:

> > A mirrored 2x36 setup will probably yield a marginal hit on writes (vs a
> > single disk) and an improvement on reads due to having two drives to read
> > from and will (based on the Scientific Wild Ass Guess method and knowing
> 
> slightly offtopic:
> 
> Does anyone one if linux software raid 1 supports this method (reading from
> both disks, thus doubling performance)?

Yes, it does.  Generally speaking, it increases raw throughput by a factor 
of 2 if you're grabbing enough data to justify reading it from both 
drives.  But for most database apps, you don't read enough at a time to 
get a gain from this.  I.e. if your stripe size is 8k and you're reading 
1k at a time, no gain.

However, under parallel load, the extra drives really help.

In fact, the linux kernel supports >2 drives in a mirror.  Useful for a 
mostly read database that needs to handle lots of concurrent users.