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August 25, 2008

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Storagezilla

I had to laugh when I saw that paper dragged out of it's cold grave but it's NetApp SOP in such cases.

When they can't compete on product alone they go for stunt paperwork.

Stephen McDonald

Hi Scott,

VTL's don't stripe to all array sets at once. Inbound IO streams are directed to one raid group unless there is a performance benefit to switching groups. What you are stating is the worst case and not the average case.

Scott Waterhouse

Yep, as I discussed above, I just used Alex's numbers for how many RAID groups are, on average, used by a virtual tape. His own chart (a little suspicious to me, but I went with it) shows it is from 2-4 RAID groups per virtual cartridge. So what I am stating is the actual case, using NetApp's data. If he wants to come back and say that the data is wrong, fine. But I am just using what he put out there. I stand by the analysis on that basis: NetApp poses from 135-270% greater chance of data loss per RAID group than an equivalently sized standard RAID-5 group. (Start reading after point #3 for the next 4 paragraphs.)

Alex McDonald

I'm not quite following on the arguments you make on my table.

http://blogs.netapp.com/shadeofblue/2008/08/i-blame-the-wea.html

The average is 1 RAID group per cartridge.

Take the first row;

. Virtual Tape Volumes (304)
. 2 RAID groups (10)
. 3 RAID groups (0)
. 4 RAID groups (0)
. >4 RAID groups (0)

and the difference (294 cartridges) on 1 RAID group.

Because of space constraints, I didn't show the "Maximum RAID Groups Used per Virtual Tape Volume" for a count of 1, but I assumed it would be clear.

Obviously not.


Scott Waterhouse

It was not. :)

But that is OK. At the end of the day, all the nice folks over at NetApp continue to label RAID-5, and by extension any single parity RAID, as professional malpractice. All explanations taken into account, the data shows there is no substantive difference between VTL RAID and any other single parity RAID in terms of rebuild times, reliability, etc. In short, all the criticisms that NetApp levels against single parity protection apply to the Near Store VTL.

If the reliability and risk associated with single parity is acceptable is an individual subjective evaluation, and probably only a part (small part?) of the evaluation that would select a VTL. But at the end of the day, NetApp at least has a consistency issue, a worst has an issue of responsibility (why sell something you really believe is dangerous?) and an issue of reliability to the extent that the need for dual parity protection on a VTL with 1 TB drives is real.

Alex McDonald

Better talk to Chuck Hollis then. Apparently RAID5 (8+1) is good enough for Exchange, a Tier1 application. We're consistent in our message, yours is all over the place.

http://chucksblog.typepad.com/chucks_blog/2008/08/your-storage-mi.html

Scott Waterhouse

The only way you could possibly be consistent is to withdraw the Near Store VTL from the market (because it is professional malpractice to sell single parity large drive systems) or admit that in some cases, single parity is appropriate.

My apologies if the utilization discussion wasn't helpful, however, try this out for irony: the only way to mitigate the risk with 1 TB drives to a level equivalent to what you claim for rebuild times (1/3 of the actual time) of your dangerous VTL RAID implementation would be to ensure that capacity never went above 34%. Maybe it isn't only filers that have a problem...

OK, relax, that was a joke. True, but funny.

Finally, since a necessary part of your argument is that for single parity RAID to be dangerous it must use large drives, and since the implementation discussed around Exchange and RAID-5 *does not* use large drives, this is, in classic NetApp fashion, another red herring.

Alex McDonald

"Finally, since a necessary part of your argument is that for single parity RAID to be dangerous it must use large drives, and since the implementation discussed around Exchange and RAID-5 *does not* use large drives, this is, in classic NetApp fashion, another red herring."

Good point, and worth exploring in a lot more detail. I'll address this as soon as I can. I write my blog out of company hours, and time is short right now due to travel and non-work stuff, and I want to get the second part of the RAID-DP piece out of the way first.

I'll be back!

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