Raid5 (Attn: Art Kagel)

[ART KAGEL, BLOOMBERG/ 65E 55TH]

On Monday, December 8, 2003 11:11, John Miller wrote:
On Monday, December 08, 2003 10:27, ART KAGEL, BLOOMBERG/ 65E 55TH wrote:
> > It has become apparent to me, based on the under informed
> > recommendations and assertions being made recently to people requesting
> > guidance about how to structure their disk arrays, that I need to post
> > my NO RAID5 ranting here as I have in other places.
<snip>
> > To put things into perspective: If a drive costs $1000US (and most are far
> > less expensive than that) then switching from a 4 pair RAID10 array to a 5
> > drive RAID5 array will save 3 drives or $3000US.  What is the cost of
> > overtime, wear and tear on the technicians, DBAs, managers, and customers
> > of even a recovery scare?  What is the cost of reduced performance and
> > possibly reduced customer satisfaction?  Finally what is the cost of lost
> > business if data is unrecoverable?  I maintain that the drives are FAR
> > cheaper!

> Thanks, Art -- that's as good an analysis as I've ever read.

> It also makes a compelling business case, but what, in your opinion, is
> optimum for a home installation where the *integrity* of RAID is desired, but
> at minimal cost (keeping in mind that many of us are more than willing to
> trade transactions per second for ultimate integrity).

> Software- or hardware-based RAID, and what configuration?

At home, for casual use, the monitary argument is less critical and indeed the 
actual cost of additional drives becomes important since the opportunity cost of
 the additional drives may be household or personal essentials.  This case is 
different from a business situation and the reason I tend to stay out of these 
discussions on most newsgroups/lists.  In the case of today's post, I noticed a 
definite business bent to the discussion so I joined in.  For home use, I'll 
assume you are planning to use the structure for filesystems, rather than for a 
database like Oracle, DB2, Informix, etc.  The performance hit from RAID5 is not
 nearly as significant for filesystem accesses as it is for databases' page 
oriented IO into their own cache.  

So, what to recommend... If the raw money is not an issue I would still go for 
RAID10 for home simply because a home system is LESS likely than a business 
system to have a valid and timely backup available and safely stored.  If one is
 confident about taking backups regularly and especially of one of the 
journaling filesystems is used and the journal database is kept on a different 
structure, then the savings from RAID5 might be worth the risk.

Art S. Kagel