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Thanks Art.
If I'm understanding you correctly, I should check to make sure that the extra capacity is coming from more stacked platters, and not just more tracks. ( that is probably the case ).
And even then I will suffer some small decrease in performance as tables fragment.

But, it is very important to continue to put data on these larger disks by striping it the way we have been, with raid and plaid.

Is that about what you said ?

Thanks !!



----- Original Message -----
Subject: Re: disk drive selection question
From: "Art Kagel" >;art.kagel@gmail.com>
Date: Thu, May 8, 2008 9:00







Floyd,

Here's the deal.Â* First, sustained read/write speeds assume sequential IO so the width of the platters does not affect that spec regardless.Â* Second, I assume these are all 3.5'' or all 5.0'' drives, not different sizes.Â* For the most part, the drive manufacturers increase the capacity of their drives by adding more platters not be increasing the number of tracks, so increased capacity in the same drive product line (say Seagate Barracuda drives) will all have the same basic performance characteristics.Â* Here's where there will be a difference between using 36G drives and 146G drives.Â* If you are replacing 4 or 5 striped 36G spindles with a single 146G spindle to get the same capacity, then there will be a 75-80% reduction in overall performance because you are only reading or writing that 58-96mb/sec from/to one spindle instead of from/to 4 or 5 spindles at once.Â* For a single small write that would hit only one drive in the array you won't see any difference, but for
larger IOs, like at checkpoint time or when sequential scanning a large table, the difference will be HUGE.

If you are, on hte other hand, moving from say 5 36G drives to 5 146G drives in the same stripe then no, there will not be any performance penalty to get the 4 or 5-fold increase in storage capacity under normal circumstances.Â* The only caviat is that when your tables become fragmented, the detrimental effect can be a bit greater than on the smaller drives over time simply because there will necessarily be more head movement to gather the data from the larger drives since there will be more nooks and crannies for the extra extents to hide in.

Art S. Kagel
Oninit



We are currently running OLTP on and Ibm P570 aix 5.3 attached to a DS4500 san. We currently have 36g 15k drives, Fiber Channel disks in there.

We are running out of space, and drawers. Our sales people are telling us that we should move to 146g 15k rpm drives.
The supposed specs on those drives, from an ibm white paper are:
3.5ms to read, 4ms to write, max sustained transfer rate 58-96mb/sec.

The funny thing is, they have the same performance rates whether the drive is 73g or 146g.
We have been under the impression that the bigger a drive is, the slower the average IO will be, due to the fact that the head has to do more moving around to get a bigger part of the data.

So my questions basically are:
1) Is our premise correct, or is IBM correct in saying that we will not suffer a performance decrease by moving to bigger drives ?
2) Has anyone done any testing on these ? I wouldn't be sure where to begin to do my own benchmarking, without having a totally isolated system. Or am I just reading too much into this and should just buy the bigger disks.
3) What am I missing here ?

Any advice is appreciated.

Thanks,
Floyd