[geeks] Big Damn File System....

[Gregory Leblanc]

OK, I missed the beginning of this, but...

> -----Original Message-----
> From: David Murphy [mailto:drjolt+geeks at redbrick.dcu.ie]
> 
> Quoting <Pine.SOL.3.95.1010622085901.1634C-100000 at wilkes>
> by "Joshua D. Boyd" <jdboyd at cs.millersville.edu>:
> 
> > On Fri, 22 Jun 2001, David Murphy wrote:
> > 
> > > > ... Which is precisely why you use RAID5. :-) I had the 
> misfortune of
> > > > loosing one hard drive of a two drive stripe set.
> > > 
> > > 2 drive RAID5?
> > 
> > Maybe he broke the two drives into two partitions each, 
> then raid5ed the 4
> > partitions.
> 
> I suspect that would have horrible performance. 2 disk RAID5 I would
> expect to have better performance, but I would still expect 2 disk

You really don't want to have more than 1 portion of a single RAID array on
a single disk.  First, this will cause some tremendous thrashing, and
second, it defeats the purpose of having a Redundant Array of Inexpensive
Disks, as it kills your redundancy.
As for performance...  A 2 disk mirror should have equal read performance to
a 3 disk RAID 5 array.  The more disks in the level 5 array, the faster
reads are.  For writes, RAID 1 is slightly slower than a single disk,
depending on how well the RAID code and the hardware controller work
together.  RAID 5 has some inherent latency in the writes, while the stripes
and the parity for those stripes are calculated.
RAID 5 isn't a level that you choose for performance, you choose it if cost
and redundancy are an issue.  For performance, choose a straight stripe set,
and for performance coupled with redundancy, choose RAID 10.  I always have
to think and figure out whether that's a stripe of mirrors, or a mirror of
two stripes...  Later,
	Greg