Re: Oracle 10gR2 / RAID-5 / SAN

On Dec 21, 1:43 am, jeremy <jeremy0...@xxxxxxxxx> wrote:
On Dec 20, 10:06 pm, joel garry <joel-ga...@xxxxxxxx> wrote:





On Dec 20, 8:02 am, jeremy <jeremy0...@xxxxxxxxx> wrote:

Hi folks

Any pointers to any papers on the pros and cons of this kind of
configuration?

thanks

--
jeremy

In addition to baarf,http://kevinclosson.wordpress.com/?s=raidhttp://www.freelists.org/arc...

I've spent a lot of time on small RAID-5, and it does work.  However,
it is amazing how often things that are not supposed to happen do:
loss of more than one disk, saturation of I/O, controller
configuration issues, etc.  There used to be a bunch of papers about
vendors supporting small raid-5 hardware on oracle.com, when there was
some initiative to push it.  I had the sense that initiative was
marketing-driven, something having to do withhttp://www.oracle.com/partners/home/why_allusers/en/benefits.html.

You should at least get a commitment to allow some other configuration
for critical files that do better in other RAID configurations, should
they prove to be a problem.  From what I've seen, these would be
serial write intensive files like redo, archived redo and backups.
Also undo, simply because it is the most hit tablespace on the OLTP
types of db's that I tend to work on.  Since the backups and other
batch type jobs tend to happen after hours, no one much cares about
the I/O saturation as long as things complete in some reasonable time.

When it comes down to it, I take space over speed, if the speed isn't
enough, someone else will drive the political push for it.  I just
make sure I'm doing the best with what I have.

I have been looking around for more info on this and came across:

http://spiralbound.net/2006/09/08/why-modern-raid-5-is-ideal-for-orac...

Here is an excerpt:

"There is a convention of thought amongst Oracle DBA's that databases
should never be installed on disks that are configured into a RAID 5
array. The argument goes, that since Oracle accesses and writes to
random points within relatively large files, the overhead of
constantly calculating block-level parity on these files is
substantial, resulting in serious performance degradation. They
suggest that RAID 1 (mirroring) is the ideal disk configuration since
no parity needs to be calculated, and Oracle is more than happy to
divide up its database over many smaller mount points.

This way of thinking has largely been correct over the years because
most systems have traditionally used software RAID. This means that
the CPU of the server itself had the job of doing all those parity
calculations, and it really did slow down both the server and the disk
when RAID 5 configurations were used. Oracle, in particular, had a
hard time with these configurations for the exact reasons the DBA's
point to.

In many cases, software RAID is still used, and to be sure, it is
wholly inappropriate to deploy RAID 5 in these environments. However,
it is increasingly common to find IT departments using a SAN-type
architecture where the RAID type and configuration are invisible to
the host operating system. In these environments, the the disk array
has a dedicated controller that is singly tasked with handling all
read, write, and parity operations. The RAID controller is no longer
software running on a generic CPU, but rather firmware that is
optimized to handle parity calculations. This results in a system
where parity is calculated so quickly by the dedicated controller that
differences in speed between RAID 1 and Raid 5 should be virtually
nonexistent."

So.... does this alter the views of folk here? Reading this it would
suggest that the issues of writer performance may be mitigated by the
SAN managing everything.

Well, yeah, and that is why I mentioned saturation and controller
configuration issues. Everything is fine until it isn't. At some
point - and the point is unpredictable given the chaotic nature of
increasing transaction volume in a real world system - there is a
severe divergence between RAID 1 and RAID 5. Since hardware is so
"cheap," it isn't all that hard to just throw a bunch at it, and if
you don't run into a wall, and someone doesn't do anything stupid like
believe MTBF figures or just put things together inappropiately (like
single-pathing the controller), they're equivalent. Then the business
grows and it goes to hell much earlier, and you can't do much because
by then the SAN is fully populated and your capital budget is
exhausted.

Those darn walls can hurt if you haven't protected your delicate
parts. Don't just CYA, wear full body armor with an extra-thick
codpiece.

jg
--
@home.com is bogus.
Someone call Jan Hammer! http://www.physorg.com/news117283969.html
.

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