Sean Cummins on 12 Dec 2008 12:48:25 -0800

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Re: [PLUG] Dell Mini 9 loses 2 GB SSD

On Fri, Dec 12, 2008 at 2:00 PM, James Barrett <> wrote:
On Fri, Dec 12, 2008 at 10:15 AM, Art Alexion <> wrote:
> I think I understand now.  I use fdisk to create a 16 GB partition, but with
> mkfs I only create a 14 GB file system.  Is that what you are saying?

Yes, that is what I was trying to say.  I do not know for a fact if
that is what is happening, however.

That sounds like a pretty good theory to me -- that the filesystem isn't occupying 100% of the space available in the partition.

Just thought I'd also point out that the 16GB SSD is probably a bit less than 16GB... It's standard practice for marketing folks in the storage industry to publish capacity numbers using "marketing" / base 10 numbers.  They take the total available space on the drive in bytes, and divide by 1,000 three times (and then generally round up a bit) to produce the capacity in GB.  But to derive the true usable capacity in base2/"engineering"/GiB numbers, you need to divide by 1,024 instead of 1,000.  The difference isn't all that significant for small drives.. but it's becoming much more noticable with larger drives (1TB+), and large arrays (100s-1000s of TBs).

To get a good feel for what the real usable capacity will be, take the number in GB and multiply by 1000 three times, then divide by 1024 three times.  This isn't quite exact due to the marketing roundup factor, but it should be pretty close.  E.g. the 300GB FC drives that EMC sells -- the marketing number is really 299.76 (which is derived from the cylinder count on the drive).  The engineering number (GiB) is 279.17.

Without knowing the cylinder count/size or the real marketing number, you can do a rough marketing->engineering conversion --  300*1000*1000*1000=300000000000 / 1024/1024/1024 = 279.39GiB -- that's very close to the actual usable capacity of 279.17GiB.

So for the 16GB SSDs... 16*1000*1000*1000=16000000000 /1024/1024/1024 = 14.9GiB.  Subtract the Dell 256MB partition from this, and you have 14.65GiB remaining.


- Sean

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