Linked by Thom Holwerda on Fri 19th Oct 2012 20:07 UTC
Windows Interesting little tidbit from the Reddit AMA session with Microsoft's Surface team. One Redditor wondered just how much disk space Windows RT takes up - in other words, if you buy the 32GB Surface RT tablet, how much space is left for your stuff? It turns out that while Windows 8 RT is considerably smaller than its Windows 7 x86 predecessor, it's still huge by mobile standards.
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RE[2]: Not quite true
by Laurence on Sat 20th Oct 2012 01:37 UTC in reply to "RE: Not quite true"
Member since:

And it's not even 12 GB. It's actually 9.8 GiB.

I can't believe nobody has pointed this out yet.

What Microsoft actually said on that Reddit thread was that "After the OS, OfficeRT and a bunch of apps, you will still have more that 20GB." Thus, they did not say that disk usage was 12 GB. They said that free space was 20 GB.

Ah, but how did he get that free space figure? Probably in Windows Explorer -- which reports free disk space in binary gibibytes, even though it uses the "GB" abbreviation. Yet flash memory is sold using decimal gigabytes. My "128 GB" SSD is reported as "119 GB [= GiB]" in Windows Explorer.

A 32 GB flash drive = 29.8 GiB. If you have "more than 20 [GiB]" of free space remaining, then that means the entire running system takes up 9.8 GiB of space, not 12.

AFAIK SSDs do actually report the space correctly. What actually happens is not the entire SSD volume is available to fill; some is always reserved for load wearing (thus extending the lifetime of the unit).

The reason being that SSDs use a CoW method (copy-on-write) and if you had a full SSD, then there's less free block to write each fs update too. Thus you're forced to recycle the same blocks (which is very bad for the lifetime of solid state drives). However as modern SSDs keep a little bit of space back, it means there's a greater pool of free blocks to balance the writing across.

So a 120GB SSD does actually have 120GB of storage but you're only allowed to fill ~120GB as the remainer is there purely for load wearing.

I hope that makes sense, i should have been in bed 2 hours ago so it's probably not the clearest post I've made lol

Reply Parent Score: 2

RE[3]: Not quite true
by tanzam75 on Sat 20th Oct 2012 02:02 in reply to "RE[2]: Not quite true"
tanzam75 Member since:

So a 120GB SSD does actually have 120GB of storage but you're only allowed to fill ~120GB as the remainer is there purely for load wearing.

That the 7% overprovisioning factor happens to be the same as the difference between GiB and GB is a nice coincidence. But it's just that -- a coincidence.

The coincidence disappears when you look at drives that have RAISE. See, e.g., the 960 GB OWC Mercury Electra MAX 3G SSD:

It has 1024 GiB of raw capacity, and 894 GiB of formatted capacity. The difference is not 7% but 14% -- and it's because some of the raw capacity is reserved for RAISE as well as overprovisioning.

They're advertising this as a 960 GB drive (= 894 GiB), not as a 1024 GB or a 1 TB drive. In other words, they're not advertising the raw capacity in binary prefixes, but the formatted capacity in decimal SI prefixes.

Magnetic hard drives also had a higher raw capacity than formatted, as did floppy disks. Because the difference wasn't anywhere close to the 5% difference between MiB/MB or the 2.4% difference between KiB/KB, the urban legend did not have a chance to spawn as it did with SSDs.

(Trick question: Why are 1.44 MB floppies called that? They're neither 1.44 decimal MB, nor 1.44 binary MiB, nor 1440 decimal KB, nor 1440 binary KiB.)

Edited 2012-10-20 02:15 UTC

Reply Parent Score: 3

RE[4]: Not quite true
by Brendan on Sat 20th Oct 2012 08:55 in reply to "RE[3]: Not quite true"
Brendan Member since:


As far as I know, all magnetic storage manufacturers have always used "dodgy numbers" - a combination of binary and decimal. A "dodgy MB" is 1024*1000 bytes, which is slightly larger than a real MB and slightly smaller than a real MiB. A "dodgy GB" is 1024*1000*1000 bytes, a "dodgy TB" is 1024*1000*1000*1000 bytes, etc.

To add to the confusion, there's "unformatted capacity" (what the magnetic material is capable of), "low level formatted capacity" (what the magnetic material is capable of storing after it's been split up into sectors) and "formatted capacity" (how much free space you're left with after you've put the overhead of a file system on it).

For a simple example; a "1.44 MB" floppy disk has an unformatted capacity of about 2 MB, a "low level formatted" capacity of exactly 1440 KiB, and (depending on which file system you use) this probably drops to about 1300 KiB of free/usable space once a file system is slapped on it.

Of course everything else used for storage (RAM, file sizes, etc) is typically measured in binary sizes (e.g. 8 GiB of RAM); and there are still many morons that use decimal prefixes for binary sizes (e.g. 8 GB of RAM); so it's natural for people to assume a dodgy TB is a binary TiB and wonder why they've been ripped off. In this case the manufacturers like to pretend that file system overhead is the only cause, instead of admitting that their dodgy number scheme is a deliberate scam.

Now; the difference between "dodgy" and "decimal" is constant (e.g. a decimal MB is 2.4% smaller than a dodgy MB, and a decimal PB is 2.4 smaller than a dodgy PB). However, the difference between "dodgy" and "binary" increases with scale (e.g. a dodgy MB is 2.4% smaller than a binary MiB, and a dodgy PB is 9.95% smaller than a binary PiB).

This means that as hard drives get larger, the "dodgy sizes that people naturally assume are a binary sizes" scam grows. For example, by the time we get to yottabytes people will be getting 18% less disk space than they assume they're getting.

The simplest solution is to hunt down these morons that use decimal prefixes for binary sizes and "re-educate them with extreme force". Once people get used to binary sizes they'll start assuming that a dodgy MB is a real MB; and then people will get more disk space than they assumed they would from hard disk manufacturers.

TL;DR: The problem can be solved with violence! ;)

- Brendan

Reply Parent Score: 5