Re: Raid
From: Steve Foster [SBS MVP] (steve.foster_at_picamar.co.uk)
Date: 06/18/04
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Date: Thu, 17 Jun 2004 18:07:02 -0700
root wrote:
>
> "Steve Foster [SBS MVP]" <steve.foster@picamar.co.uk> wrote in message
> news:xn0djn1am6zcjnq006@msnews.microsoft.com...
> > root wrote:
> >
> > > > The rest you would place according to performance requirements
> > > > remembering that the mirror set will give you faster performance
> > > > for reading
> > >
> > > No, RAID 5 is faster on reads.
> >
> > You've stated that fallacy on more than one occasion.
>
> No, just the facts. Try and cite a source supporting your position.
Why don't you? You're the one making the claims.
>
> > Simple fact is that read performance will vary depending on server
> > usage. RAID5 might be faster than RAID1, but it doesn't necessarily
> > follow that it will be.
>
> Apples vs apples >3 drive RAID 5 will always read faster than 2 drive
> RAID 1.
>
> > Bear in mind that in RAID5, the parity information must also be
> > read,
>
> FALSE!
Really? So how does the RAID controller know that the read data is
correct if it doesn't read the parity information too. What on earth
would be the point of calculating and writing parity data if it's not
going to be used?
>
> > so more data than needed must be read.
>
> Do you actually know anything about how RAID 5 works?
Do you?
>
> > Bear in mind that in RAID1, two read requests can be satisfied
> > simultaneously by applying one to each half of the mirror.
>
> Right.
>
> > RAID5 cannot
> > do that - each request must be satisfied in turn.
>
> Flat WRONG. RAID 5 could have an independent read going on EACH
> drive in the array simultaneously and under load WILL.
Ahem, in RAID 5, each block of data is spread across multiple drives,
so to complete any read request on RAID5, all drives must be read
together. This is how RAID5 manages to survive any one drive failing -
because both the data and the parity information is spread across all
drives.
>
> > Bottom line, a RAID5 array with just 3 or 4 drives is unlikely to
> > outperform a RAID1 mirror under general I/O conditions.
>
> Bottom line is that you obviously have little experience/knowledge
> regarding storage technology.
Bottom line, you really have no clue.
>
> > However, in situations where the data requested is all contiguous
> > and in large quantities, a RAID5 array probably would beat a RAID1
> > mirror, even with just 3 drives.
>
> Well, >3 drive RAID 5 will be faster that 2 drive RAID 1 in small
> record random I/O situations as well.
No, because the read requests must be queued on RAID5, where they can
be executed in parallel on RAID1.
>
> > Given that an SBS is a general-purpose server, RAID10 probably
> > offers the best all-round performance,
>
> Disreputable jump to RAID 10, I see.
And why would mention of RAID10 be "disreputable"? I guess you're not
comfortable with using it, ergo, it must be a bad choice.
>
> The best all around performance in RAID 0 but that is anti-reliable.
> RAID 5 performs like RAID 0 on reads but with one fewer drives.
What is this drivel? RAID5 has the overhead of parity information,
which is not the case with RAID1 or RAID10.
>
> RAID 10 is excessively expensive in that it requires double the
> number of drives.
That depends on your POV. But the question of cost is not directly
relevant to a discussion of performance. Now if we want to start
talking about price/performance ratios and cost considerations, then
yes, we can start talking about RAID10 being more expensive than RAID5
or RAID1.
>
> > RAID1 probably next best, and RAID5
> > probably comes 3rd.
>
> RAID 5 is the preferred solution when one needs more than one drive
> except where there is a specific WRITE performance requirement that
> one is willing to pay the RAID 10 premium to get.
RAID5 is one possible solution. It may be *your* preferred solution in
all cases, but I prefer to weigh the merits of each depending on the
situation.
>
> Study up on storage technology.
Study up on social skills.
-- Steve Foster [SBS MVP] --------------------------------------- MVPs do not work for Microsoft. Please reply only to the newsgroups.
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