Monday, August 31, 2009

How much disk space did Snow Leopard really save?

Like apparently hundreds of thousands of others, I upgraded my machines running Mac OS X from version 10.5 (Leopard) to 10.6 (Snow Leopard) last Friday. I'm now a Snow Leopard user, and I like it just fine.

I was excited about this upgrade, because I love the notion that the people who released it care about optimizing the performance of my system. One of the optimizations I looked forward to was reclaiming over 6 GB of disk space after the upgrade (see Bertrand Serlet's announcement at 00:20:48 to 00:21:11 in the WWDC 2009 keynote video).

Lots of people in the Twittersphere were excited about the space savings, too. Before I upgraded, I checked to see what people were tweeting, just to make sure I wasn't about to walk off a cliff. Many people mentioned tremendous disk space savings that were well in excess of the 6 GB that Apple promised. Pretty exciting.

We have two Mac computers. Here's how the savings went for us:
Mac #1      10.5      10.6      Savings
------- --------- --------- ---------
Total 148.73 GB 159.70 GB
Free 47.70 GB 59.71 GB 12.01 GB

Mac #2 10.5 10.6 Savings
------- --------- --------- ---------
Total 185.99 GB 199.71 GB
Free 68.66 GB 83.95 GB 15.29 GB
So,, we saved over twice as much space as Apple had advertised. But there's a curiosity in the numbers. Do you see it? How did my total capacity get bigger as the result of a software upgrade? The answer is that my capacity didn't really get bigger; it's just that Apple now measures disk space differently in 10.6.

I knew this was coming because of this article called "Snow Leopard's New Math." Snow Leopard still uses the abbreviation "GB" to refer, now, to 109 bytes, whereas, before, Leopard used the abbreviation "GB" to refer to 230 bytes. The problem, see, is that 109 ≠ 230. In fact, 230 is bigger. So in Snow Leopard, Apple is dividing by a smaller unit than it used to, which results in disk capacities and file sizes looking bigger than they used to. (Here's a good article about that.)

It is misleading that Apple used the same abbreviation—"GB"—to refer to two different units of measure. However, Apple is well justified in using "GB" in Snow Leopard. IEEE 1541-2002 says the right abbreviations would have been "GiB" (gibibytes) in 10.5 and "GB" (gigabytes) in 10.6. By that standard, Snow Leopard is right, and Leopard was wrong. All's well that ends well, I suppose.

Now, back to the space savings question. How much space did I really save when I upgraded to Snow Leopard? To answer that, I need to convert one of the two columns in my analysis (labeled "10.5" and "10.6") to the other column's unit, so I can subtract. Since when I watched the WWDC keynote film, my mindset was of 10.5-style "gigabytes" (properly gibibytes), I'll convert to GiB. Here's the answer:
Mac #1      10.5        10.6      Savings
------- ---------- ---------- ---------
Total 148.73 GiB 148.73 GiB
Free 47.70 GiB 55.61 GiB 7.91 GiB

Mac #2 10.5 10.6 Savings
------- ---------- ---------- ---------
Total 185.99 GiB 185.99 GiB
Free 68.66 GiB 78.18 GiB 9.52 GiB
That's still spectacular, and I'm plenty happy with it. I have basically bought a whole bunch of performance enhancements and 17 GiB of disk space for $49 plus tax (I bought the Snow Leopard upgrade family pack). I think that's a pretty good deal.

This whole story reminded me of the old days when I used to install Oracle for a living. People would buy, say, a brand-new 100,000,000-byte disk drive and then be upset when the df utility showed considerably less than 100 "MB" of free space. Part of the explanation was that df reported in mibibytes, not millions of bytes.

It's interesting to note that in Snow Leopard, df -h now reports in Bi/Ki/Mi/Gi units, and df -H reports in B/K/M/G units (defined as IEEE 1541 defines them). Smart.


Jake said...

I'm really digging Snow Leopard, but I have my fair share of qualms as well. No more cut and pasting Address Book contacts??? Check the rest out here:

Cary Millsap said...

Thanks, Jake. Your complaint about copy & paste for Address Book is one of the things that has always bugged me on the Mac. It doesn't seem any different to me now than it was before the upgrade. Where were you previously able to click to get the whole address in one copy? —Cary

Jake said...

Hey Cary. I was actually alway able to pull up a contact in Address Book and start at the bottom or top and highlight the entire contact to copy and paste it elsewhere. Everyone in my office was also. It was never formatted perfectly as you'd have to delete the spaces between "mobile" and telephone number or "work" and the email address, etc, but at least you could throw it all into an email at once. Weird....

If everyone was savy enough you could just send vcards, but half the smart phone and computer users out there don't know what to do with a vcard when sent. Bummer!

Big Bill Tolbert said...

That's real nice but since I supported Apple three years ago by buying the PowerPC chip I'm screwed!

Cary Millsap said...

Ah, the old "Early Adopter Needs A NuttyBuddy" syndrome :-).

Michael Katz, A10 Networks NYC said...

yo dude, nice work!

Jake said...

Hey Everyone -

Pop back to my blog if you'd like for a decent temporary work around for the address book copy and paste problem.

You can copy and paste off of the print preview screen (print -> preview).


Jared said...

The GiB vs GB designation has always smacked of weasel based marketing by the drive manufacturers.

Converting between GiB and GB has always been a pain. If the folks that made the drives had just used 2^30 ( actually 2^20 a few years ago) it would have saved us all a lot of trouble.

I have a small capacity planning app that I converted to use just 10^9 so it would match space reports for network systems.

Cary Millsap said...

It's a trick that a manufacturer can play only once, isn't it.

Brian Tkatch said...

GiB/GB It's a case where marketing argues with practice. Practice that has been in place for tens of years. And, the people that use it, means exactly the opposite of marketing.

I wonder if they use the metric system at home?

At least they list the actual size somewhere as well.

Skipjacker said...

I found this comment to be interesting, "I love the notion that the people who released it care about optimizing the performance of my system."

I think the people who released it did what they could to transfer $49 from as many people as possible to themselves. If the goal was as benevolent as you say, wouldn't it be free? Because they chose to make 'features' of the new OS include performance enhancements, and your very positive reaction toward that choice, is more a sign that Apple understands its owners' g-spots more than than anything. In fact, the very idea that you see this as a commitment to optimization shows how much you're willing to drink the kool-ade. I have a feeling if this were a new feature of another commercially available OS, your attitude would be along these lines: "They've been wasting 7 GB of my drive for years and now they've finally fixed *that* bug."

Cary Millsap said...


I think you're right about everything you've said here, except that I didn't mean to imply that I think Apple are benevolent (or that I think they should be). I am pleased that Apple's decisions about what software features to release are in alignment with my priorities, and I'm pleased to support those decisions with my cash.

Now that you point it out, I don't like the way I worded that sentence in my blog post. I should have written something like this instead:

"I was excited about this upgrade, because it seems that its design priority is optimization, which is a priority to me. Having lived in the Microsoft Windows world for so long, it is refreshing that a new software release would be marketed dominantly upon the attribute of its being cleaner than its predecessor."

Jeff said...

Hi, Skipjacker.

There are a number of ways to define benevolence. As far as I can tell, they all mean the act of helping others. Some definitions even allow for some type of profit on the part of the party rendering the help.

So, to your point, I don't see why we can't consider Apple's action as benevolent.

Especially, considering the low cost of the upgrade.

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