Inside System Storage -- by Tony Pearson

Tony Pearson Tony Pearson is a Master Inventor and Senior IT Specialist for the IBM System Storage product line at the IBM Executive Briefing Center in Tucson Arizona, and featured contributor to IBM's developerWorks. In 2011, Tony celebrated his 25th year anniversary with IBM Storage on the same day as the IBM's Centennial. He is author of the Inside System Storage series of books. This blog is for the open exchange of ideas relating to storage and storage networking hardware, software and services. You can also follow him on Twitter @az990tony.
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Tony -<div>&nbsp;</div> Credible-sounding overview, but I think you may have stretched the truth with your marketecture again:<div>&nbsp;</div> * The RAID rebuild times you quote undoubtedly represent best-case, sequential rebuilds with no competing workload on the damaged RAID group. Disk drives are much faster at handling fully sequential work queues than they are random ones- I doubt you can demonstrate 5 hour rebuilds of a 750GB drive on any storage platform you sell, at least not while also supporting application workloads during the rebuild.<div>&nbsp;</div> * By extension, I suspect that the recovery you describe takes longer than the "3-4 minutes per wave" that you hypothesize because each drives' workload looks more like random I/O than sequential, especially if the primary production workload doesn't stop while the lost drive is recovered.<div>&nbsp;</div> * 750GB = 750,000 1MB blocks, so the loss of a 90% full drive means 700 THOUSAND 1MB blocks now have no matching mate (I think you just dropped a zero there somewhere).<div>&nbsp;</div> * Your attempt to justify the expense of Mirrored vs. RAID 5 makes no sense to me. Buying two drives for every one drive's worth of usable capacity is expensive, even with SATA drives. Isn't that why you offer RAID 5 and RAID 6 on the storage arrays that you sell with SATA drives? <div>&nbsp;</div> And if RAID 5/6 makes sense on every other platform, why not so on the (extremely cost-sensitive) Web 2.0 platform? Is faster rebuild really worth the cost of 40+% more spindles? Or is the overhead of RAID 6 really too much for those low-cost commodity servers to handle.<div>&nbsp;</div> Or perhaps Moshe already convinced you all to "Mirror Everything?"<div>&nbsp;</div> * You skimmed over it, but the fact remains that ANY second drive failure before the recovery from the first one will almost always result in data loss, a probability that increases with the size of the drives and the percentage that each of the drives are full (because it takes longer to rebuild them all). Only RAID 6 can protect against a double drive failure; nothing you've described mitigates this risk of data loss.<div>&nbsp;</div> * I understand why you might think that this all scales linearly, but I sincerely doubt that it does. At a minimum, the larger the drive community that LUNs are spread across, the higher the probability the system will experience multiple concurrent drive failures - no matter HOW fast the data from each lost drive is rebuilt.<div>&nbsp;</div> * Finally, it'd be real nice to hear from those 40-plus "customers" you keep referencing (although I suspect that they'd be more accurately referenced as "Alpha Testers"). I'm sure many of us would be interested to know if they put them into actual production use, if they ever came anywhere close to using 90% of the capacity -and if so, how was performance and rebuild, and which specific "Web 2.0" applications they were using the Nextra for.<div>&nbsp;</div> Don't get me wrong - it's an extremely interesting architecture. You just haven't convinced me (at least) that it's "magical" yet.

2 localhost commented Trackback

BarryB,Good catch, yes, I was off by a zero, it was 700 thousand, but the rest of the numbers are correct, based on IBM DS8000 rebuild times. I guess the rebuild times could be slower on non-IBM devices, but that is beside the point.<div>&nbsp;</div> The point is not RAID-1 or RAID-5 per se, as much as doing it at the object level rather than the drive level. Even RAID-6 is not enough to protect very large drives, as the possibility of a bit flip increases, and the intervening cache could get hit by cosmic rays during the rebuild process; but RAID-6 could be done at the object level, with 14 blocks having 2 parity blocks, but again, hardware is just a small piece of the total cost of ownership.<div>&nbsp;</div> The 90% utilization was to ensure there is free space available for rebuilds. I doubt anyone in production runs higher than this. Running down at the traditional 30-50 percent would also qualify as "having enough free space".<div>&nbsp;</div> The "magic" is separating the blocks from the drives. Call it "internal virtualization" if you like. Separating the logical from the physical is how the magic happens.

3 localhost commented Trackback

I can't help but follow up to this phrase in your response to my comment: "Running down at the traditional 30-50 percent would also qualify as "having enough free space". "<div>&nbsp;</div> At 50% utilization, with mirrored storage, you're using 75% more physical storage than your actual data consumes. No matter HOW you cut "cost of ownership," that's a HUGE premium to pay: whether you look at acquistion, operational, or energy costs.<div>&nbsp;</div> Look around you, Tony- all of your competitors are implementing thin provisioning specifically to drive physical utilization upwards towards 60-80%, and that's on top of RAID 5/RAID 6 storage and not RAID 1. Given that disk drive growth rates and $/GB cost savings have slowed significantly, improving utilization is mandatory just to keep up with the 60-70% CAGR of information growth.<div>&nbsp;</div> So, that you would propose that a Web 2.0 company should buy 3x ADDITIONAL storage for their Web 2.0 data (not including remote replicas and complete backups, just in case of a double-drive failure)?<div>&nbsp;</div> Well...I'll just say this: it shouldn't be too difficult to compete with THAT approach!

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I am looking for the IBM VM Poster or a picture of the IBM VM "Catch the Wave" <div>&nbsp;</div> Do you know where I might find it

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