ext4 blocksize peculiarities

ext4 blocksize peculiarities

[Bernhard Rohrer]

Hi all

I have a 3.3 TB partition (yes really) that is formatted with ext4 and 4 
kiB blocks. What little data I have on that appears to use up an 
extraordinary amount of space, so I had a closer look.

root@newcollab:/home/adminlion# df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/mapper/primary_vg-primary_var
                       3.5T   43G  3.3T   2% /var

we see that 43 GiB appear to be taking up something that gets rounded to 
200 GiB

if we look at:

root@newcollab:/home/adminlion# df
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/mapper/primary_vg-primary_var
           3696706360  44362800 3464561624   2% /var

now 44362800 4k blocks are 169.23 GiB, which does indeed round to 200 
GiB. similar usage also gets reported when I get the properties of the 
share via cifs.

debugfs shows:

Block count:              937689088
Reserved block count:      46884454
Free blocks:              912175341

so while df shows 4 times as many available 1k blocks as debugfs, which 
after all shows the actual number of 4k blocks, both are more or less in 
agreement as to the space used. This leads me to believe that somehow 
only 1k of every 4k block gets used. Now i could of course reformat to 
1k blocks, but I'd prefer to avoid that.

OS is Lucid64 and the volume is a LVM volume.

Has anybody seen this before? Uncle Google was rather mum on the subject.

cheers

Bernhard

Re: ext4 blocksize peculiarities

[Eric Sandeen]

Bernhard Rohrer wrote:
> Hi all
> 
> I have a 3.3 TB partition (yes really) that is formatted with ext4 and 4
> kiB blocks. What little data I have on that appears to use up an
> extraordinary amount of space, so I had a closer look.
> 
> root@newcollab:/home/adminlion# df -h
> Filesystem            Size  Used Avail Use% Mounted on
> /dev/mapper/primary_vg-primary_var
>                       3.5T   43G  3.3T   2% /var
> 
> we see that 43 GiB appear to be taking up something that gets rounded to
> 200 GiB

by default 5% is reserved for the super-user, 5% of 3.3T is 168G,
168G + 43G is roughly 200G

-Eric
 
> if we look at:
> 
> root@newcollab:/home/adminlion# df
> Filesystem           1K-blocks      Used Available Use% Mounted on
> /dev/mapper/primary_vg-primary_var
>           3696706360  44362800 3464561624   2% /var
> 
> now 44362800 4k blocks are 169.23 GiB, 

those are 1k blocks :)

Anyway, what you are seeing is the 5% reserved space, which does
not show up in "Used" but is reflected in "Avail"

It is confusing and you're not the first one to get tripped up :)

-Eric

> which does indeed round to 200
> GiB. similar usage also gets reported when I get the properties of the
> share via cifs.
> 
> debugfs shows:
> 
> Block count:              937689088
> Reserved block count:      46884454
> Free blocks:              912175341
> 
> so while df shows 4 times as many available 1k blocks as debugfs, which
> after all shows the actual number of 4k blocks, both are more or less in
> agreement as to the space used. This leads me to believe that somehow
> only 1k of every 4k block gets used. Now i could of course reformat to
> 1k blocks, but I'd prefer to avoid that.
> 
> OS is Lucid64 and the volume is a LVM volume.
> 
> Has anybody seen this before? Uncle Google was rather mum on the subject.
> 
> cheers
> 
> Bernhard
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