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Revision as of 16:59, 20 November 2012


Welcome to the explain xkcd wiki! We already have 4 comic explanations!

(But there are still 1733 to go. Come and add yours!)

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Datacenter Scale
Asimov's Cosmic AC was created by linking all datacenters through hyperspace, which explains a lot. It didn't reverse entropy--it just discarded the universe when it reached end-of-life and ordered a new one.
Title text: Asimov's Cosmic AC was created by linking all datacenters through hyperspace, which explains a lot. It didn't reverse entropy--it just discarded the universe when it reached end-of-life and ordered a new one.

Explanation

Ambox notice.png This explanation may be incomplete or incorrect: The paragraph explaining the final panel needs a grammar check.

This comic expands, to the limit, the strategy that it's a net cost savings to allow cheap hardware to fail and simply replace it than to have robust but much more expensive systems to start with. The technique was made famous by Google circa 1999, when its successful cost-effective server designs were actually using sub-consumer, nearly junk, hardware.

RAID ("redundant array of independent disks") is a technology that splits data across several hard-drives as if they were one. RAID comes in several levels (varieties) which have different applications, but one of the big applications of RAID is creating mirrored hard disks that back each other up. If one disk drive in such a RAID fails, no data is lost.

However, RAID is complicated to configure, so you don't want to be constantly setting it up. An alternative technique for data centers is therefore to simply send the data to several servers at once. This makes maintenance easier, but without RAID, one hard disk crash basically breaks the server. However, this is what the woman with a bun's (possibly an adult Science Girl) data center is doing since their scale is so large that fixing individual servers does not make sense, and instead of fixing the drive they throw away the machine.

From here, the comic starts to exaggerate. Nowadays, servers can be made extremely small ("Blade servers") and dozens of servers can be attached to one 19-inch rack in a data center. Rather than going to the effort of unplugging and unscrewing one blade from the rack, when a blade fails at Cueball's data center they just throw away the rack, and Ponytail agrees and kinda mocks the woman with a bun for replacing one server.

Hairy's goes one step further - they have so many servers that they would constantly have to be throwing away and replacing racks, so instead they just build a new room when one rack fails. This would be currently possible with small modular data centers that are built in shipping containers for easy transport and can be linked together to expand capacity. Here the cargo-container "room" with the failure would be quickly swapped with a fresh one. Cueball adds "like Google!" - Randall previously mentioned Google's approach to hard drive failures in the what if? Google's Datacenters on Punch Cards. Back in 2007 they had one failure every few minutes - that might have increased hugely since then.

Finally Megan appears and her company of course breaks the scale of silliness in exaggeration. She says that they don't have any fire extinguishers (neither regular sprinklers nor the inert gas systems that flood the room with gases like argon which force out oxygen and stop fires - these are better for computers than traditional water or powder systems, but are lethal to people trapped in the rooms!). Rather, they just rope the center off, thus letting the data center burn down. Then they simply move a town over and build a new one. This may indicate they are so big that the entire town will burn down if their center catches fire, for else they did not have to skip town. Alternatively, they just leave the center burning and this may cause problems in that town, so they simply flee the premises.

Most big internet companies do have multiple redundant data centers around the world, in order to increase speeds for users in different countries, but Megan's idea would be very expensive, increase in latency and possibly also kill people, either in their company or other people in the town and since they do not try to turn out the fire, at least cause severe destruction of properties, not only their own.

Still Hairy thinks that it makes sense, whereas Cueball wonders what difference the roping off does. This could again be a reference to the fact that they just let the buildings burn without bothering about the local consequences, and the next step is just one more step towards the extreme of the title text.

This comic references how, as data requirements expand, the cost of time eventually outweighs the cost of hardware at ever increasing scales (drive, rack, room, building). While this comic takes this to the extreme, with whole buildings being destroyed for simple flaws, the concept is not as far fetched as it seems if "thrown out" is taken to include being sold to equipment refurbishers. It could indeed be cost effective for a large data services provider to resell racks or even whole data center modules at some significant fraction of their "as new" price as opposed expending the time and effort to attempt a repair. The equipment refurbisher would then rely on a cost advantage like cheaper labor to repair the flaw and sell it back to Google or another company with less demanding requirements. Equipment rental firms already operate on this model and with the added incentive customers preferring to rent newer models, this means that the equipment is often preemptively replaced before failures even occur.

The title text refers to Isaac Asimov's science-fiction short story "The Last Question", where humanity asks, at different stages of its spatial and technological development, the same question to increasingly advanced computers: "How can the net amount of entropy of the universe be massively decreased?". At each point, the computer's answer is that it does not yet have enough data for a meaningful answer. Ultimately, the computers are all linked through hyperspace, outside the physical boundaries of the universe, and make up a single computing entity named AC which keeps pondering the question even as the heat death of the universe occurs and time and space cease to exist. When AC finally discovers the answer, since there is nobody left to report it to, it decides to demonstrate it and says "LET THERE BE LIGHT!", which are the first words said by God during the Creation, according to the Book of Genesis. Here, the title text implies that, as the universe died, AC no longer had a use for it as a physical support and, taking the comic's logic to the next extreme, chose to discard it and get a brand-new one instead of bothering to "fix" it by reversing its entropy.

This comic's concept of taking a real world phenomena and exaggerating it to levels currently considered implausible for comic effect closely mimics an earlier comic which describes progressively more "hardcore" programmers in 378: Real Programmers.

Transcript

[Zoom in on a woman with a bun holding her hand palm up in front of her taking to people off-panel right.]
Woman with a bun: RAID controllers don't make sense at our scale; everything is redundant at higher levels. When a drive fails, we just throw away the whole machine.
[In this frame-less panel it is revealed that the woman with a bun talked to Cueball and Ponytail who is looking her way.]
Cueball: Machine? We throw away whole racks at a time.
Ponytail: Yeah, who replaces one server?
[Hairy has appeared from the left and holds one hand palm up towards the other three where also the woman with a bun has turned towards him.]
Hairy: We just replace whole rooms at once. At our scale, messing with racks isn't economical.
Woman with a bun: Wow.
Cueball: Like Google!
[Megan walks in from the left, and everyone including Hairy now looks towards her. Cueball has taken a hand up to his chin. The replies to Megan are written with clearly smaller font.]
Megan: We don't have sprinklers or inert gas systems. When a datacenter catches fire, we just rope it off and rebuild one town over.
Hairy: Makes sense.
Cueball: I wonder if the rope is really necessary.


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