RE: RAID (was: Is VMS losing the Financial Sector, also?)
- From: "Paul Raulerson" <paul@xxxxxxxxxxxxxx>
- Date: Sun, 8 Jul 2007 17:27:29 -0500
This is different experience on my part. I usually have multiple redundant
paths to storage, and multiple controllers in a host. As in, it does not
matter if you are shadowing or using RAID-5+, if you only have two
controllers and both of them go down, so does the host.
I don't usually use host based direct control of storage, because enterprise
class storage systems rarely go all the way down and *do* have massive
amounts of cache in them. They usually are populated with very hefty
processors too, and lots of those.
The "it" I referred to was host based shadow capability. And you are right
about the cost. In trying to configure a small system, it is near impossible
to control cost. What in an enterprise system is a quite reasonable cost, is
in a departmental sized (or even smaller) server, quite a ridiculous cost.
There are cases where the license to do something costs quite a bit more
than the entire hardware system. (*sigh*)
Just another obstacle to be traversed. :)
I am impressed with VMS being able to traverse almost the entire range
though; small servers to enterprise global configurations.
Sweet.
-Paul
-----Original Message-----
From: Ken Fairfield [mailto:Ken@xxxxxxxxxxxxxxxxxxxxx]
Sent: Sunday, July 08, 2007 5:06 PM
To: Info-VAX@xxxxxxxxxxxx
Subject: Re: RAID (was: Is VMS losing the Financial Sector, also?)
Paul Raulerson wrote:
Just curious- with hardware raid why did you really even consider> shadowing? Raid-5 would give you the same benefits and data
> protection as well. If in a hot-swap array, it also means zero
> downtime for your system.
Actually, hardware RAID-5 does *not* give the same benefits and
protection as host based shadowing!
First, hardware RAID, by definition, introduces a single point
of failure (not just from a power outage, but also through various
storage system failures that can cause *both* controllers in a
dual-redundant configuration to become unavailable (been there,
seen that, more than once). HBVS can be/should be configured with
no single point of failure.
Second, RAID-5 has performance issues (mostly hidden in modern
systems by massive caches) that can be problematic if/when you
lose a RAID member. HBVS does as well during a shadow copy
following a member failure, but current releases of the shadowing
software (fully patched 7.3-2 and above) make that almost
invisible except in rare cases. I've omitted merge impacts
because those transient are very to extremely short these days.
In a modern day shop, which enterprise class storage systems, I can> not see any need at all for shadowing. :)
I couldn't disagree more... :-( Keith Parris had a good write
up several years ago about why he came to consider dual-redundant
controllers, and storage-based RAID and mirrorsets, a single point
of failure. For enterprise class installations, availability has
to be a primary concern, and single points of failure must be
avoided wherever possible.
Of course, it makes sense on a low end system though, and is a verycool
capability built into the system. For the life of me, I cannot figureout
why RAID-0 was considered an add on product and RAID-1 was a baseproduct
though. :)
I don't quite understand the first sentence above; what does the "it"
refer to??? As to the rest of the paragraph, and if we're referring
to VMS, shadowing (RAID-1) always required a license that was none too
cheap. The fact that HBVS was/is integrated into the system was more
of an engineering issue than anything else (not so easy to split off
all its tentacles into a separately packaged product). This is the
old, "why is DECNET an integrated product (SIP) while TCPIP Services
is a layered product?" Only the engineers can say... :-) It's not
as surprising that Host-based striping (RAID-0) is a layered product.
It depends upon the shadowing software already being present and just
adds one more layer on top of that.
(As an aside, I seem to remember that on HSC40's, controller based
mirroring was part of the basic software, but you needed to purchase
at extra cost a license to enable the controller based striping.
I have no idea why DEC did that. I also never saw an HSC40 with
the striping option!)
It's interesting that the "I" in RAID _used_ to stand for
"Inexpensive".
Now days, the cost of the storage controllers/RAID controllers is
quite a bit more than that of the individual disks. (IIRC, last time
I bought storage, the cost of the cabinets plus HSG80 pairs was more
than the cost of the all disks they were hosting...but that will vary.)
Regards, Ken
--
Ken & Ann Fairfield
What: Ken dot And dot Ann
Where: Gmail dot Com
.
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