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Old 06-08-2008, 03:25 PM
"Mag Gam"
 
Default RAID for large disks

Well said.

Thankyou and everyone


On Sun, Jun 8, 2008 at 10:14 AM, Damon L. Chesser <damon@damtek.com> wrote:

On Sun, 2008-06-08 at 07:33 -0400, Mag Gam wrote:

> Again, I appreciate the responses.

>

> Damon:

>

> I am dealing with HW RAID. I looked for the "geometry" for my

> controller, but could not find it.

>

> http://h20000.www2.hp.com/bizsupport/TechSupport/DocumentIndex.jsp?contentType=SupportManual&lang=e n&cc=us&docIndexId=64179&taskId=101&prodTypeId=329 290&prodSeriesId=1157686


>

> I am very curious about the geometry too...

>

> *I don't know enough to pick and choose the optimal setting. Since I

> am working in the academic field, I would like to really understand

> this "geometry setting". Can someone please elaborate on this topic?

>

> TIA



Mag,



It looks like your controller does not let you set very much manually:

page 13 shows you can set the Stripe Size. * At this point, jump in and

play. *But again, I don't think it will amount to a hill of beans. *The

controller "masks" all reads and writes to the physical drives and the

OS is ignorant of the underlining details. *IF you needed to set it up a

certain way, you would KNOW it. *And even so, it looks like the only

things you can adjust with this controller is the stripe size. *In

almost all cases the "optimal" setting is the default of the controller

when you use it's setup "wizard" thingy, what ever it may be called.

All other deviations are for very specific instructions in some manual

for some application (or you spend much time bench testing to arrive at

what works best for you with that given hardware). *Some advice: *unless

you are being graded in some way on the maximum through put of this

server: *Jump in, install it and be done.



We (IT) don't know the "optimum" settings of such things. *We play with

it per some set of (specific) directions or we have a test box we can

bench test to meet some objective with. *This concept is a moving,

slippery thing. *It all depends on your network throughput, latency, cpu

load, bus load, I/O of every other component, I/O of the controller,

it's (the controller) memory, physical hd read/write speed and probably

a few more I can't think of right now.



Kill this beast, install the os using defaults for the HDs, see if you

can serve up the files at a rate that works for you, if not, look it

over again.



BTW, on the next model server you get, everything you learn here will

not be valid unless it is the exact same set of hardware. *That is why I

would not fret over this, there is no "great maxim" to be learned except

"can I set up THIS box to work at the rate I need?" *Sometimes the

answer is no, but that falls onto the procurement end of the deal.



A DBA will spend much time telling a sys admin what strip to put onto a

RAID (using a hardware controller), but that is from the application mfg

having benched tested a specific model of server with very specific

hardware to arrive at the best throughput possible with a given hardware

load out. *This is not your situation. *The only answer is to test.



Anyway, that is my 2C worth.





--

Damon L. Chesser

damon@damtek.com

http://www.linkedin.com/in/dchesser
 
Old 06-09-2008, 01:56 AM
Luke S Crawford
 
Default RAID for large disks

"Mag Gam" <magawake@gmail.com> writes:
> I want RAID 5 but without mirroring. The data is important but not that
> important.

Ok, there are performance advantages and disadvantages to RAID5.
First, the advantage: reading is awesome. almost as good as a stripe.

the other advantage: writes in full stripe-size increments are also pretty
good... it's as good as a stripe minus the calculation overhead (calculation
overhead is pretty low these days, especially if you have a hardware
raid)


The downside:

writes less than the stripe size suck. the thing is, each (16K in your
case) stripe needs to be calculated as one unit so the parity stuff works.

so if you write, say, 8K, it needs to read in the 8K of the stripe you are
not writing, combine that with the 8K you are writing, recalculate parity
and write the full 16K stripe.

(write-back cache can solve this problem; but write-back cache,
in most cases, is also pretty dangerous. as another poster said, make
sure your raid card has battery backed cache before enabling write-back
cache)

So if your stripe size is larger than your average write, things suck
mightily.

> If the controller creates a stripe size of 16k, do I need to do anything
> special with physical extends (in pvcreate or vgcreate) ?
> Do I need to do anything specific when creating a LV? I plan on striping my
> LV to create extra spindles. Do I need to create my ext3 filesystem with any
> particular settings? I am looking for a optimal tuning guide with emphasis
> on performace versus redudancy.

'striping my LV to create extra spindles' - I don't understand.
the performance-enhancing spindles are physical disks... partitioning
a disk and pretending that each partition is a physical disk isn't going
to help you.


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