Re: RAID

[lsorense@csclub.uwaterloo.ca (Lennart Sorensen)]

On Thu, Nov 10, 2005 at 09:57:02AM -0800, lordSauron wrote:
> On 11/10/05, Albert Dengg <a_d@gmx.at> wrote:
> > well it has to map the raid device to the real discs...like with all
> > raid levels
> >
> > also...
> > raid 0 isnt really raid...
> > raid means _redundant_ array of independet/inexpensive discs...
> > and raid 0 isnt redundant at all
> 
> I'm interested in RAID 0 b/c I want drive speed, not drive
> reliability.  I've got a full-tower ATX case which runs amazingly cool
> and quiet, so I'm not concerned with my drives freaking out and
> spontaniously dying.
> 
> So the current installer does not support the motherboard's RAID,
> which is slower, but what about setting up linux's kernel's RAID?  Can
> the current Sarge installer do that?
> 
> Also, I know about RAID 0, 1, and 50, but what on earth is RAID 5 and
> 6?  I think RAID 5 has to do with networked JBODs, but I'm not sure...

RAID0 (not raid) is stripping data over multiple disks to increase
performance.

RAID1 is mirroring data across 2 disks to increase reliability (at the
cost of half your disk space).

RAID 3 4 and 5 are stripping with parity across multiple devices to
increase speed and reliability, although it requries xor calculations
liek there is no tomorrow and hence often is only done with a hardware
xor engine for acceleration.  Modern CPUs with good MMX/SSE/etc
algorithms are not bad at it either though.  Raid 5 stripes the parity
data across the disks to avoid a heavy load on a single disk storing
parity data.  Raid 3 and 4 store the parity data on one device only.
Costs you one disk worth of disk space out of your total space (so you
can 700G if you have 8 * 100G drives in raid 3/4/5).

RAID6 is RAID5 with ECC.  It stores a second set of parity data to allow
error correction.  It costs you two disks worth of space out of the
total (so you get 600G if you have 8 * 100G drives).  It can I believe
tolerate 2 disk failures without data loss (although at that point you
get no error correction anymore).

Supposedly from what I have seen, RAID10 is disks stripped, then mirror
two identical size RAID0s, while RAID01 is disks mirrored, and then
stripe the mirrors, and RAID50 is two RAID5s mirrored.

Len Sorensen