Subject: Re: RAID: which level to use? (documentation slightly lacking)
To: None <>
From: Thor Lancelot Simon <>
List: tech-kern
Date: 04/22/1999 17:09:56
On Thu, Apr 22, 1999 at 10:05:09PM +0200, Leo Weppelman wrote:
> On Thu 22 Apr 1999, Erik E. Fair wrote:
> > I've read the raid(4) and raidctl(8) man pages, and neither really answers
> > this simple question:
> > 
> > I have two disks, I want to be protected against one of them failing, so
> > some sort of mirror or redundancy is required. It looks like RAID level 1
> > (mirror), or RAID level 5 (striped with parity distributed over components)
> > are the obvious possibilities; what are the advantages and disadvantages of
> > each?
> In a nutshell:
>   raid1: safe and (relatively) fast but not space effective.
>           - safe against failure of 1 of the 2 disks
>           - fast in read (can be distributed among drives), a write
>             must be done twice...
>           - You loose 50% of space

With disk drive prices as cheap as they are today, generally you see
"raid 0+1", where you build either a stripe of mirrored components, or
a mirror of striped components.  This generally outperforms any other RAID
solution in the absence of special-purpose hardware to accellerate the
parity computation of RAID4 or RAID5.

But something is terribly wrong with RAIDframe's mirroring code.  Its
striping (RAID 0) code is only about 5-10% slower than our old 'ccd'
device, but a mirror of two 7200RPM SCSI disks for me writes only at about
20-40KB/sec!  I don't consider that useful.