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* Posts by Destroy All Monsters

9436 posts • joined 3 Jun 2008

Why does our galaxy spiral?

Destroy All Monsters
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Why am I playing Summly?

> there are more things than stars in galaxies were these included in the simulation

From the paper:

The galaxies in our study consist of dark matter halos and rotationally supported disks of stars. The parameters describing each component are independent and the models are constructed in a manner similar to the approach described in previous works (Hernquist 1993; Springel 2000; Springel et al. 2005).

2.2. Dark Halo

We model the dark matter mass distribution with a Hernquist (1990) profile:

rho_dm = M_dm/ (2*Pi) * a / (r * (r + a)³)

which has a cumulative mass distribution M (< r) = M_dm * r² / (r + a)² , where a is the radial scale length and M_dm is the total halo mass here set to 9.5 * 10¹¹ M_solar.

In the past, models of disk galaxies run in isolation and used to study the properties of spiral arms employed only a few million particles to sample both the stellar disk and the dark matter halo. In such experiments, randomly-placed particles produce fluctuations in the halo potential. Even if the disk is initially featureless, the Poisson noise owing to such discretization of the mass in N-body experiments is inevitably swing amplified, producing trailing multi-armed spiral patterns in the disk (Toomre 1977; Fujii et al. 2011; Sellwood 2012).

In order to suppress the development of artificial features in all the N-body experiments that follow, we set up a live disk of stars embedded in a rigid dark matter potential. We employ simulations with a sufficiently large number of particles in the disk, i.e. 100 million, so that the disks are essentially featureless when evolved without any perturbers acting on them. These simulations serve as “controls,” making it possible to identify the response of the disk to imposed perturbations. In this manner, we will be able to separate the sources responsible for exciting features in the disk from the stars which react to the perturbations, unlike previous experiments in which the stars themselves acted as perturbers, complicating the interpretation of the experiments, as emphasized by Toomre (1990).

Looking forward to gas + full stellar evolution simulations for added beauty.

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Destroy All Monsters
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Re: Interesting simulation

From the paper:

A long-standing controversy over the nature of spiral arms is whether they correspond to density enhancements in the background stellar distribution (density waves), or are made up of stars that always remain in the arm and are just more concentrated than the stars outside the arm (material arms). In the early studies, the arms were assumed to be density waves, because if they were material they would quickly wind up as the galaxy rotates. Thus, both the swing amplification and the static density wave theories argue that the arms are overdense regions of the disk moving around at a different speed relative to the stars themselves. Stars thus continuously move in and out of the spiral arms. However, recent investigations using numerical simulations of stellar disks have challenged this claim and and argue that the arms might be material structures (Grand et al. 2012). To investigate this in the context of our simulations, we identify a patch of stars along the arm in the stellar disk after the arms are fully developed (after 100 Myrs) as displayed in top panels of Fig. 6, where the patch is colored in black. Then, we follow the positions of the stars originally in the patch forward in time and display the outcome after two galactic years (bottom panels). We note that the stars initially in the patch spread out, confirming that the spiral patterns in our simulations are density waves and not material structures. This is shown in polar coordinates in the bottom panel of Fig. 6 where it is clear that the patch is being sheared out by differential rotation and the pitch angle of the patch differs significantly from that of the spiral features.

Looking forward to Matt Bryant complaining how Suns are shit.

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Destroy All Monsters
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Paris Hilton

Re: Waste of time and money

You must have been one of those differently abled kids with special needs?

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Re: Ye cannae change the laws of physics

More at the Galaxy Zoo

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Monkey poo probe reveals secrets of middle-management brains

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Holmes

Re: What this study mainly proves..

Are you hailing from the top or from the bottom?

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'Proactive Wellness' rebranded as 'Infosight' by Nimble

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Production-ready ZFS offers cosmic-scale storage for Linux

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Holmes

Re: Destroyed All Braincells, Maximum Damage Control

Why should I indulge you?

These "convenient cartoons" can be easily had these days btw. Just use google image search.

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Destroy All Monsters
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Trollface

Re: Destroyed All Braincells Gordon Gordon Phil Gordon Gordon AC Destroyed All .....

Maximum Damage Control

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Destroy All Monsters
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Re: Michael Wojcik A more important ZFS feature

A Conversation with Jeff Bonwick and Bill Moore, September 1, 2007

BILL MOORE We had several design goals, which we’ll break down by category. The first one that we focused on quite heavily is data integrity. If you look at the trend of storage devices over the past decade, you’ll see that while disk capacities have been doubling every 12 to 18 months, one thing that’s remaining relatively constant is the bit-error rate on the disk drives, which is about one uncorrectable error every 10 to 20 terabytes. The other interesting thing to note is that at least in a server environment, the number of disk drives per deployment is increasing, so the amount of data people have is actually growing at a super-exponential rate. That means with the bit-error rate being relatively constant, you have essentially an ever-decreasing amount of time until you notice some form of uncorrectable data error. That’s not really cool because before, say, about 20 terabytes or so, you would see either a silent or a noisy data error.

JEFF BONWICK In retrospect, it isn’t surprising either because the error rates we’re observing are in fact in line with the error rates the drive manufacturers advertise. So it’s not like the drives are performing out of spec or that people have got a bad batch of hardware. This is just the nature of the beast at this point in time.

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Destroy All Monsters
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Coat

Re: zpool scrub == fsck

But was it labeled AE-35?

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Destroy All Monsters
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Re: Must say it..

Your NEEDS will GROW. ZFS can help!

“We're in the process of receiving two visitors from Earth.” Gisela was astonished. “Earth? Which polis?” “Athena. The first one has just arrived; the second will be in transit for another ninety minutes.” Gisela had never heard of Athena, but ninety minutes per person sounded ominous. Everything meaningful about an individual citizen could be packed into less than an exabyte, and sent as a gamma-ray burst a few milliseconds long. If you wanted to simulate an entire flesher body — cell by cell, redundant viscera and all — that was a harmless enough eccentricity, but lugging the microscopic details of your “very own” small intestine ninety-seven light years was just being precious.

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Destroy All Monsters
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Facepalm

Re: Michael Wojcik A more important ZFS feature

When that "mythical" bit-rot blows away a good chunk of your encrypted filesystem, you will be singing from another sheet - and from another hole.

Seriously, I sure hope responsible people keep you in the cellar, away from anything critical. The way you are sounding off, you must be pulling a good spiel and bedazzle quite a few of the people with the purse strings, so probably not.

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Destroy All Monsters
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Trollface

Re: Gordon Gordon Phil Gordon Gordon AC Destroyed All Braincells Gordon BTRFS?....

Matt, just stop it ... mmm'okay. There is a good boy. Take a deep breath, watch some heist movie, open a bottle of fine red for tonight.

I'm getting flashbacks to the scenes of rabid retardation of clueless testosterone-filled nerds in the uni computer room attempting one-upmanships in the Commodore-vs-Atari flamewars via local chatrooms.

Once two of them found out they were actually sitting in the same room, and it came to blows...

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Destroy All Monsters
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Holmes

Re: XFS not safe

"ZFS on linux is production ready? Hmmm...."

It's always a question of: how much production readiness do you want?

Is it production-ready enough? I guess so. You left out this little thing in the middle of the text, written in 2009:

"In the case of ext3, it's actually an interesting story. Both Red Hat and SuSE turn on barriers by default in their Enterprise kernels. SuSE, to its credit, did this earlier than Red Hat. We tried to get the default changed in ext3, but it was overruled by Andrew Morton...."

Is the sky falling? Evidently not. Is it getting better? Yes! And seriously, is there anyone (except the ones who like to riding bikes where their unprotected balls 5 are cm from the tarmac) who uses ReiserFS?

Also:

BARRIERS ON BY DEFAULT IN EXT4: YES PLEASE and Enabling/Disabling Write Barriers in Fedora 14

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Destroy All Monsters
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Thumb Up

Re: A more important ZFS feature

Cool.

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Destroy All Monsters
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Devil

Re: Gordon Phil Gordon Gordon AC Destroyed All Braincells Gordon BTRFS? You must...

Woah epic thread.

Give it up Matt, you are just coming across as a juvenile angry neckbeard with mysterious chips on the shoulder.

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Destroy All Monsters
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Re: Destroyed All Braincells Gordon BTRFS? You must be joking...

Why insist on truck-lifiting distributed filesystems into the discussion? Who mentioned them first? And what is a "Sunshiner"?

> Trying to bolt together a Frankenstein network filesystem out of Luster and ZFS seems pretty pointless when Red Hat already have a superior option in GlusterFS, or hp with LeftHand.

Apparently some people who imply that they are "in the industry" actually believe that a distributed filesystem is a magic black box that drops off a vendor's conveyor belt, glittering in pixie dust, instead of basically a bunch machines, each with disks arrays having their own filesystem plus a remotely accessible centralized lock manager and possibly a metadata server.

A place in marketing beckons. The BS levels are there. Bonus for mentioning "FTSE 1000" and some HP products.

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Destroy All Monsters
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Holmes

Re: A more important ZFS feature

> the number of times I've seen this lately make me never want to touch a non-checksummed FS again

Well, the disk itself checksum its blocks, so how can the errors be silent?

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Destroy All Monsters
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Headmaster

Re: Must say it..

> The classical mechanical degrees of freedom of a proton (disregarding the quarks and gluons that compose such a particle) in 3d space are surprisingly enough are 3.

They clearly would be 9: 3 for the position, 3 for the momentum, and 3 for the axis of rotation.

> However one could precisely measure the position of such a particle.

No. Because you need too much energy to do that.

> by zero point fluctuation alone.

Explaining impossibility to determine position and momentum at the same time is better explained by the olde Fourier Transform: peaks in frequency domain mean flat functions in time domain and conversely and doesn't involved Marvel Comics Science Terminology.

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Destroy All Monsters
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Facepalm

Re: Gordon BTRFS? You must be joking...

Knowledge Matt® now in a new edition of: "Undistributed filesystem is undistributed".

Film at 11.

Also: "ZFS is suitable for desktops and that's about it."

Words fail. These must be *big* desktops.

You can check out some of the desktops here. They are evidently using ZFS on backend nodes to provide a clustered filesystem on top. Hmm.....

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Destroy All Monsters
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Holmes

What the hell?

> Add the usual suspects delivering "unsupported packages because lawyas" (like DeCSS, MP3 decoders etc.) to your list or recognized package sources.

> yum install

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Animal Liberation drone surveillance plan draws fire

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Trollface

...and then you get downvoted, too!

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Destroy All Monsters
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Trollface

Re: Daily Mail

What! Atheist an AGW believer, too! My Goodness! What is this civilization coming to.

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Destroy All Monsters
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Trollface

Re: Daily Mail

Are you some kind of leftist?

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Destroy All Monsters
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Only in Australia: The country tries to kill you while the Animal Liberation Front is looking after your cattle.

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Destroy All Monsters
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The vache of Louis de Lombard doesn't rit, Monsieur!

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Destroy All Monsters
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Coat

Fetchez la vache.

뭐라구??

FETCHEZ LA VACHE!!

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Destroy All Monsters
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Holmes

"With cattle trucks lying in wait ... for the next time" (Pink Floyd)

> film of Australian cattle being treated inhumanely

That would be inbovinely, shurely?

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Facebook prepares to dominate Android

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Devil

Yet another thing that will keep the progressives awake at night!

This just in

Experiment Shows How Facebook Can Spread Racist Thoughts

By Tom Jacobs, Pacific Standard

"Heavy Facebook users are more likely than those who log on occasionally to react positively to racist remarks."

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Destroy All Monsters
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Paris Hilton

Re: Facebook Phone Really

Wait! You mean there are "real people" out there? But ....

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World's first petaflops super dumped on scrap heap

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Trollface

Re: "Think of all the copper"

It sure won't be easily gypsy-accessible...

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Destroy All Monsters
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Paris Hilton

Maybe Cell power attracted soul eaters from the fifth dimension?

That would be explain glassy-eyed salesdroids. They HAD to get rid of it....

> and a simulation of the universe at a 70-billion-particle scale.

What simulation is that? I know about DEUS-FUR at 549 billion particles running on the CURIE supercomputer in France and the Millenium Simulation series with 10 billion to 348 billion particles.

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Veiny green 'scum' meteorite may be first visitor from Mercury

Destroy All Monsters
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Re: Its clearly not from mercury.

Is it ruguous or squamous?

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Patent shark‘s copyright claim could bite all Unix

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Re: Not quite as good as the Telegraph

No, that was real!

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Destroy All Monsters
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Holmes

"Mechanisms to do digestive gas exchange in unlighted environments"

Cm'on Reg... patent laywers don't do copyright claims.

Patent lawyers may be the scourge of the earth after politicians/state employees/mafiosi but they know how their bread is buttered.

"Copyright claims" are precise and too easy to disprove or to work around to bring in the hay. See the pathetic trick "copyrighted lines of signal constants and some commentary" by McBride's

Well, I guess McBride still enriched himself in his sociopathic ways using the burndown of his company as stepping stone, with the employees as useful idiots ... Wikipedia says McBride is now President and CEO of something called "Me Inc."

You couldn't make it up.

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Facebook buys Dummly from outernet prodigy Dick D'Miner

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Pint

Amazing channelling of the late Philip K. Dick!

And what happened to the kid that got showered in money by Yahoo?

Does he really exist or is he just an ad campaign?

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Public cloud will grow when experienced IT folks DIE

Destroy All Monsters
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Devil

Re: Public cloud will grow when experienced IT folks die

Turns out it is YOUR responsibility to put that in the contract with the cloud provider....

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Destroy All Monsters
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That's not even funny. That's like saying a car is just a bunch of sheet metal.

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Smartphone running 'Facebook OS' said to debut this week

Destroy All Monsters
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Trollface

In your face, book!

> Complains about vapid opinions

> Posts a vapid opinion with 4 "facts" from which only one is undubitably true.

Come on Zuck, stop the astroturfing.

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Dell directors foresee unremitting brutality in PC market

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Mushroom

One last thing....

So you wanna get back into the "value chain" biz?

Then GET RID OF WINDOWS.

License an OS. Any OS. Tune it into something agreeable with SWEAT, PAIN AND BLOOD. But get RID OF WINDOWS. Get RID OF THAT HORRIFIC INTERFACE. Get rid of THAT PILE OF JUNK that needs "ANTIVIRUS", that still doesn't know how to organize files properly in a meaningful hierarchy, is subject to the whims of some sweating overfed kids from marketing, is so NIMBY that it knows about FAT16 but not EXT2 ... in 2013, has a IN-HOUSE BROWSER THAT NEEDS IMMEDIATE REPLACEMENT, is so wobbly that you find trojans ensconced in C:\TMP, needs RESTARTS whenever something kernelish happens (and then may NEVER COME BACK) and is all-round a PITA allied to a LIQUID SANTORUM.

I know you have the cojones beaten out of you by a decade of "Customers Want Windows" sludge and brainwash from Redmond, but BY GOD, IT IS TIME TO WAKE THE HELL UP.

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Jobs' first boss Nolan Bushnell: 'Steve was difficult but valuable'

Destroy All Monsters
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Re: @Ross K (was: @Homer 1 (was:Misplaced admiration))

"40+ Years of Labor for THIS?"

I would buy it.

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Reg man bested in geek-to-geek combat - in World War 3 nerve centre

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Devil

Found on Google Maps, finally

Aaah, motherland!

There is a swastika-shaped building, too. Still pretending there wasn't collection of germanic know-how after the Mad Confrontation of The Two Collectivist Powers was over?

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Google Street View releases devastated Fukushima town tour

Destroy All Monsters
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Shrine and Visitors.

The effect of the visitors uncannily disppearing if you come from another direction freaks me out.

More people in one of the reality slices

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Destroy All Monsters
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You know how your energy bills are SO much worse than they were?

Destroy All Monsters
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Re: people don't go green to save money only!

> WE DON'T NEED TO USE POISON FOR ENERGY

Yeah, because these windmills and solar panels are growing on trees.

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Destroy All Monsters
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Facepalm

Re: You Know That Website That Publishea

Yet another one who thinks that the seating order in the French Republic's Assembly is connected to issues about rational economic calculation and voter-baiting greenfaggery.

No convinced by flimflammery? Must be right-wing!

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Destroy All Monsters
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Trollface

Re: Opinion(s)

>> those people who are actually running things

> politicians

> actually running things

Very good, sir

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Egyptian navy captures divers trying to cut undersea internet cables

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Trollface

Re: The mystery of the mysterious operatives

For a start we had no reason to

Are you writing out of the State Department?

The one that puts out stuff about Ghaddafi handing out viagra for better rape?

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Relaxed Windows 8 rules hint at smaller slabs to come

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Re: please could we all agree

> If you've only ever used Windows 8 in a VM, you've really not used it at all.

It's 2013. Fucking get with the program, guys.

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