Interpreting `btrfs filesystem show'

Interpreting `btrfs filesystem show'

[Anton Shepelev]

Hello, all

While trying to resolve free space problems, and found that
I cannot interpret the output of:

> btrfs filesystem show

Label: none  uuid: 8971ce5b-71d9-4e46-ab25-ca37485784c8
        Total devices 1 FS bytes used 34.06GiB
        devid    1 size 40.00GiB used 37.82GiB path /dev/sda2

How come the total used value is less than the value listed
for the only device?

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Re: Interpreting `btrfs filesystem show'

[Hugo Mills]

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On Mon, Oct 15, 2018 at 05:24:08PM +0300, Anton Shepelev wrote:
> Hello, all
> 
> While trying to resolve free space problems, and found that
> I cannot interpret the output of:
> 
> > btrfs filesystem show
> 
> Label: none  uuid: 8971ce5b-71d9-4e46-ab25-ca37485784c8
>         Total devices 1 FS bytes used 34.06GiB
>         devid    1 size 40.00GiB used 37.82GiB path /dev/sda2
> 
> How come the total used value is less than the value listed
> for the only device?

   "Used" on the device is the mount of space allocated. "Used" on the
FS is the total amount of actual data and metadata in that allocation.

   You will also need to look at the output of "btrfs fi df" to see
the breakdown of the 37.82 GiB into data, metadata and currently
unused.

   See https://btrfs.wiki.kernel.org/index.php/FAQ#Understanding_free_space.2C_using_the_original_tools for the details.

   Hugo.

-- 
Hugo Mills             | "Your problem is that you have a negative
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Re: Interpreting `btrfs filesystem show'

[Hugo Mills]

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On Mon, Oct 15, 2018 at 02:26:41PM +0000, Hugo Mills wrote:
> On Mon, Oct 15, 2018 at 05:24:08PM +0300, Anton Shepelev wrote:
> > Hello, all
> > 
> > While trying to resolve free space problems, and found that
> > I cannot interpret the output of:
> > 
> > > btrfs filesystem show
> > 
> > Label: none  uuid: 8971ce5b-71d9-4e46-ab25-ca37485784c8
> >         Total devices 1 FS bytes used 34.06GiB
> >         devid    1 size 40.00GiB used 37.82GiB path /dev/sda2
> > 
> > How come the total used value is less than the value listed
> > for the only device?
> 
>    "Used" on the device is the mount of space allocated. "Used" on the

s/mount/amount/

> FS is the total amount of actual data and metadata in that allocation.
> 
>    You will also need to look at the output of "btrfs fi df" to see
> the breakdown of the 37.82 GiB into data, metadata and currently
> unused.
> 
>    See https://btrfs.wiki.kernel.org/index.php/FAQ#Understanding_free_space.2C_using_the_original_tools for the details.
> 
>    Hugo.
> 

-- 
Hugo Mills             | "Your problem is that you have a negative
hugo@... carfax.org.uk | personality."
http://carfax.org.uk/  | "No, I don't!"
PGP: E2AB1DE4          |                              Londo and Vir, Babylon 5

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Re: Interpreting `btrfs filesystem show'

[Anton Shepelev]

Hugo Mills to Anton Shepelev:

>>While trying to resolve free space problems, and found
>>that I cannot interpret the output of:
>>
>>> btrfs filesystem show
>>
>>Label: none  uuid: 8971ce5b-71d9-4e46-ab25-ca37485784c8
>>        Total devices 1 FS bytes used 34.06GiB
>>        devid    1 size 40.00GiB used 37.82GiB path /dev/sda2
>>
>>How come the total used value is less than the value
>>listed for the only device?
>
>   "Used" on the device is the mount of space allocated.
>"Used" on the FS is the total amount of actual data and
>metadata in that allocation.
>
>   You will also need to look at the output of "btrfs fi
>df" to see the breakdown of the 37.82 GiB into data,
>metadata and currently unused.
>
>See
>https://btrfs.wiki.kernel.org/index.php/FAQ#Understanding_free_space.2C_using_the_original_tools for the details

Thank you, Hugo, understood.  mount/amount is a very fitting
typo :-)

Does the standard `du' tool work correctly for btfrfs?

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Re: Interpreting `btrfs filesystem show'

[Austin S. Hemmelgarn]

On 2018-10-15 10:42, Anton Shepelev wrote:
> Hugo Mills to Anton Shepelev:
> 
>>> While trying to resolve free space problems, and found
>>> that I cannot interpret the output of:
>>>
>>>> btrfs filesystem show
>>>
>>> Label: none  uuid: 8971ce5b-71d9-4e46-ab25-ca37485784c8
>>>         Total devices 1 FS bytes used 34.06GiB
>>>         devid    1 size 40.00GiB used 37.82GiB path /dev/sda2
>>>
>>> How come the total used value is less than the value
>>> listed for the only device?
>>
>>    "Used" on the device is the mount of space allocated.
>> "Used" on the FS is the total amount of actual data and
>> metadata in that allocation.
>>
>>    You will also need to look at the output of "btrfs fi
>> df" to see the breakdown of the 37.82 GiB into data,
>> metadata and currently unused.
>>
>> See
>> https://btrfs.wiki.kernel.org/index.php/FAQ#Understanding_free_space.2C_using_the_original_tools for the details
> 
> Thank you, Hugo, understood.  mount/amount is a very fitting
> typo :-)
> 
> Does the standard `du' tool work correctly for btfrfs?
> 
For the default 'physical usage' mode, it functionally does not work 
correctly, because it does not know about reflinks.  The easiest way to 
see this is to create a couple of snapshots of a subvolume alongside the 
subvolume, and then run `du -s --totals` on those snapshots and the 
subvolume.  It will report the total space usage to be equal to the sum 
of the values reported for each snapshot and the subvolume, when it 
should instead only count the space usage for shared data once.

For the 'apparent usage' mode provided by the GNU implementation, it 
does work correctly.

Re: Interpreting `btrfs filesystem show'

[Martin Steigerwald]

Hugo Mills - 15.10.18, 16:26:
> On Mon, Oct 15, 2018 at 05:24:08PM +0300, Anton Shepelev wrote:
> > Hello, all
> > 
> > While trying to resolve free space problems, and found that
> > 
> > I cannot interpret the output of:
> > > btrfs filesystem show
> > 
> > Label: none  uuid: 8971ce5b-71d9-4e46-ab25-ca37485784c8
> > Total devices 1 FS bytes used 34.06GiB
> > devid    1 size 40.00GiB used 37.82GiB path /dev/sda2
> > 
> > How come the total used value is less than the value listed
> > for the only device?
> 
>    "Used" on the device is the mount of space allocated. "Used" on the
> FS is the total amount of actual data and metadata in that
> allocation.
> 
>    You will also need to look at the output of "btrfs fi df" to see
> the breakdown of the 37.82 GiB into data, metadata and currently
> unused.

I usually use btrfs fi usage -T, cause

1. It has all the information.

2. It differentiates between used and allocated.

% btrfs fi usage -T /
Overall:
    Device size:                 100.00GiB
    Device allocated:             54.06GiB
    Device unallocated:           45.94GiB
    Device missing:                  0.00B
    Used:                         46.24GiB
    Free (estimated):             25.58GiB      (min: 25.58GiB)
    Data ratio:                       2.00
    Metadata ratio:                   2.00
    Global reserve:               70.91MiB      (used: 0.00B)

                            Data     Metadata  System              
Id Path                     RAID1    RAID1     RAID1    Unallocated
-- ------------------------ -------- --------- -------- -----------
 2 /dev/mapper/msata-debian 25.00GiB   2.00GiB 32.00MiB    22.97GiB
 1 /dev/mapper/sata-debian  25.00GiB   2.00GiB 32.00MiB    22.97GiB
-- ------------------------ -------- --------- -------- -----------
   Total                    25.00GiB   2.00GiB 32.00MiB    45.94GiB
   Used                     22.38GiB 754.66MiB 16.00KiB  


For RAID it in some place reports the raw size and sometimes the logical 
size. Especially in the "Total" line I find this a bit inconsistent. 
"RAID1" columns show logical size, "Unallocated" shows raw size.

Also "Used:" in the global section shows raw size and "Free 
(estimated):" shows logical size.

Thanks
-- 
Martin