linux-ext4 - Re: Determining if an ext4 fs uses the whole partition

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Date:	Mon, 30 Apr 2012 12:18:48 -0600
From:	Daniel Drake <dsd@...top.org>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: Determining if an ext4 fs uses the whole partition

On Mon, Apr 30, 2012 at 11:29 AM, Eric Sandeen <sandeen@...hat.com> wrote:
> For starters, use fdisk -u to get 512-byte sector units,
> otherwise it's just inscrutable CHS magic.
>
> i.e.:
>
> # fdisk -lu /dev/sda2
>
> Disk /dev/sda2: 526 MB, 526417920 bytes
> 255 heads, 63 sectors/track, 64 cylinders, total 1028160 sectors
> Units = sectors of 1 * 512 = 512 bytes
>
> so 1028160 512-byte sectors.
>
> # dumpe2fs -h /dev/sda2 | grep "Block count\|Block size"
> dumpe2fs 1.42.2 (27-Mar-2012)
> Block count:              514080
> Block size:               1024
>
> so 514080 1k blocks, or 1028160 512-byte sectors, so bingo, it's full.

Hmm yes, following the same process I can get the same results. I must
have misread/miscalculated something when I looked earlier.
User error :)

> Or, FWIW, it's harmless to invoke resize2fs if the fs already fills the
> partition; it should just exit with a no-op.

Thanks for pointing this out - its probably my best option, coming to
think of it.

>> One easy solution, if possible, would be to find out the number of the
>> last sector used by the filesystem. I could then very easily compare
>> this to the "end" information found in sysfs for the partition. Then I
>> can make the decision on whether to grow or not.
>
> dumpe2fs should certainly be able to tell you.  Mounting the fs, and
> doing statfs would, as well (f_blocks).  There should also be libext2fs
> functions you could use if you want to do it in C...

Trying the statfs approach (the fs in question is already mounted):

# dumpe2fs -h /dev/mmcblk0p2 | grep "Block count\|Block size"
dumpe2fs 1.42 (29-Nov-2011)
Block count:              949248
Block size:               4096

# stat -f /
  File: "/"
    ID: f09a7645207bdd68 Namelen: 255     Type: ext2/ext3
Block size: 4096       Fundamental block size: 4096
Blocks: Total: 934935     Free: 198205     Available: 188947
Inodes: Total: 227824     Free: 133103

The numbers don't agree.

(Not a big deal, since I can use the other 2 approaches you mentioned,
just wanted to point it out)

Thanks
Daniel
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