RAID Questions

[Brian Densmore]

> -----Original Message-----
> From: Lucas Peet [mailto:lpeet at eccod.com]
> 
> I have no experience with RAIDs.  The only thing I know is that RAID
> stands for "Redundant Array of Inexpensive Disks", and that a 
> RAID 0 basically joins
> all disks in to one 'virtual disk'.
> 
> > > It has two RAID arrays being used, one is a software array
> > > using Six Quantum
> > > 6 gb UW (20 Mb Transfer rate) SCSI drives and I have them 
> in a single
> > > striped RAID 0, to be used for backup purposes. The second
> 
> This would be 6*6= 36Gb of storage, right?  My question is:
Yes, able to use all 36gb. 

> 
> > > RAID is a RAID
> > > 0/1 using 4 60 gb IDE Maxtor ATA100 Drives and the onboard
> > > Highpoint HP370A
> 
> ### > controller (which gives me a 104 usable ext3 formatted 
> partition).###
> 
> What does he mean by this?
He should have 120gb of usable disk (N/2)*S. Where N = number of disks
and S = size of 
-=smallest=- disk. I'm not sure why he's getting 104.

> 
> > > > I want to implement RAID Level 1 (mirror) for the /boot and
> > > / partitions
> > > and RAID level 5
> > > > for the balance of my partitions.
> 
> Also, what do the other RAID levels mean?
> 
Well there are others on the list much more qualified to answer this
than I. 
But I have no fear of making myself look ____. So here goes.

RAID 0 is basically taking multiple disks look like one and uses a
striping 
mechanism to achieve some performance. There is no redundancy here for
recovering
crashed drives. Not very useful, there are many better solutions. Disk
failure all data lost?
2 drive minimum.

RAID 1 is mirrored drives. If one crashes no data is lost. This cost
speed and performance,
because each virtual drive is mirrored exactly. Highest overhead. 100%
redundancy. 2 drive minimum. N-1 drives can fail and still be able to
recover all data.

RAID 0+1 (or 0/1) This is RAID one but with RAID 0 striping on the
segments. 
This gives redundancy on a RAID 5 level, almost. Limited scalability.
One disk failure and 
array becomes a RAID 0 system until faulty disk replaced. Two disk
failure, all data lost.
Excellent for fileservers. I have seen conflicting stories on this. One
says 4 disk minimum, another says 2. I think 4 is correct.

RAID 4 two or more drives are used to store data, 1 to store parity.
Parity disk can be used to 
recreate failed disk. Not sure what happens if parity disk fails.
Requires 3 disks or more.
Complex implementation and recovery. Two drives fail all data lost.
Parity disk becomes bottleneck.

RAID 5 Like RAID 4 but data and parity is distributed among each disk.
Most difficult to recover from failed disk. One disk failure is ok. Two
disk failure all data lost. Requires 3 disks.

There are also RAID levels 2,3,6,7,10 and 53. RAID 7 is done only by CSC
they own it. I have never seen these others, although I hear RAID 6 is
the most fault tolerant allowing multiple crashes and still being
recoverable. Also very expensive to implement.

That's the way I understand RAID.

I'm sure someone will correct my mistakes,
Brian