

Why Existing Databases (RAC) are So Breakable - prabodh
http://natishalom.typepad.com/nati_shaloms_blog/2009/11/why-existing-databases-rac-are-so-breakable.html

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wmf
This article is a very poor interpretation of the facts. RAID works (as long
as it isn't RAID-5). High-end disks may have the same failure rate as low-end
disks, but the higher performance and lower capacity gives a much lower MTTR,
ultimately improving reliability.

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shrike
The author makes a huge leap,

 _"The core, implicit assumption behind Oracle RAC, IBM DB2, and many other
database clustering solutions, is that failure can be avoided by purchasing
high-end disk storage and using expensive hardware (fiber optics, etc). As can
be seen from the research I mentioned earlier, this core assumption doesn’t
correlate with the failure statistics. Hence I argue that the database
clustering model is inherently breakable."_

Taking the fact that drives fail and extrapolating that to mean that large
drive arrays are "inherently breakable" is nonsense. Using an EMC Symmetrix as
an example I am very familiar with, a large properly implemented storage array
is far from breakable. Inside that box you buy is a whole collection of n+2,
n+3 and/or n+4 components all built specifically to never, ever break. The
only problem with this type of solution is it is priced in the multi-millions.
Each.

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wmf
Jeff Darcy tears into this one: <http://pl.atyp.us/wordpress/?p=2555>

"Letting fear of conventional storage drive the creation of 'solutions' that
are just as complex without the benefits of being as general, as well tested,
or as well documented is a mistake. (Open but undocumented and untested can be
worse than closed, BTW, if the cost of reverse-engineering and fixing the
implementation is greater than the cost of licensing would have been.) Such
attempts generally lose even when considered alone, and even more so when the
effects of fragmentation and incompatibility are considered."

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ponnap
"Most existing database clustering solutions rely on a shared disk storage to
maintain their cluster state, as can be seen in the diagram below."

In the case of RAC, although state is written to a file called 'voting disk',
the actual state of the cluster is communicated through a high speed inter
connect which is redundant. Only if the network connection(s) are down, state
is exchanged through the 'voting disk'.

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Femur
One feature that this article overlooks is Oracle's Automatic Storage
Management feature (ASM) which is very commonly used with RAC. This feature is
basically a software level RAID alternative in which you can define striping
and mirroring independent of what the hardware reflects.

This article also does not really reflect what is commonly done with SAN
implementations.

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ponnap
Actually most production databases tend not to use ASM's High or Normal
redundancy because the software level redundancy provided by ASM tends to
perform very badly when compared to a storage controller driven RAID. External
redundancy is usually chosen. ASM is primarily used to easily manage the disk
groups when the volumes in the disk group tend to run out of space.

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ecq
RAC + Dataguard if you use Oracle

