difference between "df" and "fdisk" ??

crs_zxf · 06-24-2009, 06:09 AM

Hi, guys!

I wanna know why the number of 1K-blocks reported by "df" and "fdisk" are different.

For my /dev/sda7, "df" says that I have 419642824 1k-blocks, while "fdisk" says I have 426332938+ 1k-blocks (BTW, I also wonder what does the trailing '+' mean).

At first, I thought it's due to the different unit used by "df" and "fdisk", that is, to "df", 1K = 1024B, and to "fdisk", 1K = 1000B, but seems that this explanation didn't make sense, since 426332938/1.024 = 416340759.765625, which is smaller than 419642824.

So why? Any ideas? Thanks!

pixellany · 06-24-2009, 06:17 AM

fdisk reports on the disks and partitions---regardless of whether a filelsystem is installed (or which one).

df reports on the filesystem. I imagine the difference could be simmply associated with fielsystem overhead...

Curingaa · 06-24-2009, 06:26 AM

Also good to see man pages (man df; man fdisk), or also df --help and fdisk --help....

i92guboj · 06-24-2009, 06:31 AM

Quote:

Originally Posted by crs_zxf

Hi, guys!

I wanna know why the number of 1K-blocks reported by "df" and "fdisk" are different.

I guess (but not sure) that being df fs-aware, the difference strives in the fact that a fs can report less total space for many reasons, for example, space reserved for root or fs overhead as someone stated above. But, as said, not sure.

Quote:

(BTW, I also wonder what does the trailing '+' mean).

This one I can explain, fdisk counts blocks of 1024 bytes, but hardware understands sectors of 512 bytes, the + means that the volume has an odd number of sectors, hence, it has one sector at the end which doesn't form a complete block, hence, you have 426332938 blocks of 1024 bytes + one sector of 512 bytes.

crs_zxf · 06-24-2009, 08:56 AM

Quote:

Originally Posted by i92guboj

I guess (but not sure) that being df fs-aware, the difference strives in the fact that a fs can report less total space for many reasons, for example, space reserved for root or fs overhead as someone stated above. But, as said, not sure.

yeah, fs does reserve some space (normally 5% for linux) for special purposes, but the reserved space isn't reflected in the difference between "df" and "fdisk" output, it's reflected in the "df" output, in which

Code:

"Used" + "Available" < "1K-blocks"

and the difference between the two sides of the previous formula is the amount of space reserved by the filesystem.

dfezz1 · 06-24-2009, 09:08 AM

What is the actual size of the HDD? 400G or 406G ?
It has to be the filesystem verses raw blocks that is the difference. This is the only reason for the differnce in reporting.

Is this causing an issue or are you just curious? We try to fix problems here, although in doing so we do teach others and pass on aquired knowledge, but that isn't the focus.

crs_zxf · 06-24-2009, 09:25 AM

Quote:

Originally Posted by dfezz1

What is the actual size of the HDD? 400G or 406G ?
It has to be the filesystem verses raw blocks that is the difference. This is the only reason for the differnce in reporting.

The following is the entire output of "fdisk -l":

Code:

Disk /dev/sda: 500.1 GB, 500107862016 bytes
255 heads, 63 sectors/track, 60801 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk identifier: 0x624aa2e0

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *           1        6375    51207156    c  W95 FAT32 (LBA)
/dev/sda2            6376        6387       96390   83  Linux
/dev/sda3            6388       60801   437080455    5  Extended
/dev/sda5            6388        7603     9767488+  83  Linux
/dev/sda6            7604        7725      979933+  82  Linux swap / Solaris
/dev/sda7            7726       60801   426332938+  83  Linux

Quote:

Originally Posted by dfezz1

Is this causing an issue or are you just curious?

Both. Yesterday I just try to resize one of my partitions, but I'm a little bit confused about how to specify the size argument of "resize2fs" and the size argument passed to "n" command of "fdisk", resizing a partition normally involves using these 2 commands. (using "parted" is annoying, since you always need to deal with many filesystem features, like disabling some of them, which I'm not good at)

PTrenholme · 06-24-2009, 10:24 AM

There are several GUI partition editors that will hold your hand through resizing your partitions. So, if you're uncomfortable using the command line tools, try one of them.

crs_zxf · 06-24-2009, 10:34 AM

Quote:

Originally Posted by PTrenholme

There are several GUI partition editors that will hold your hand through resizing your partitions. So, if you're uncomfortable using the command line tools, try one of them.

Thanks for your advice, but I think using command line tools can make me learn more, and better understand things.

crs_zxf · 06-25-2009, 08:08 AM

Quote:

Originally Posted by crs_zxf

Both. Yesterday I just try to resize one of my partitions, but I'm a little bit confused about how to specify the size argument of "resize2fs" and the size argument passed to "n" command of "fdisk", resizing a partition normally involves using these 2 commands. (using "parted" is annoying, since you always need to deal with many filesystem features, like disabling some of them, which I'm not good at)

After doing some experiments, I think now I'm much more clear about the numbers that are reported or used by various tools.

To get a real 300G fs, you actually need to specify 304G as the argument of "resize2fs", here, by "real 300G", I mean the "size" reported by "df -h" is 300G. There's always an about 1.5% difference between the size reported by "df" and the argument you provide to "resize2fs" (of course, I don't know why, I only know the relationship between these two numbers).

Besides, the maximum amount of space that you can provide to "resize2fs" is the one reported by "fdisk -l".

Hope this info is helpful.