Minimum size of Btrfs volume ?

Minimum size of Btrfs volume ?

[Bill Dietrich]

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[Noob here, sorry if I'm doing anything wrong.]

What is the minimum size of a simple single-disk Btrfs volume,
and where is it documented ?  I can't find that info.

I'm told the minimum size is about 109 MB (114294784 bytes).
True ?  Is there any way to get around that, at mkfs-time ?
I'd like to use Btrfs inside a VeraCrypt container, and that's
a fairly big minimum size for that use.

I'm using Btrfs on Ubuntu GNOME 20.04 desktop.

Thanks,

Bill Dietrich
bill@billdietrich.me



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Re: Minimum size of Btrfs volume ?

[A L]

---- From: Bill Dietrich <bill@billdietrich.me> -- Sent: 2020-08-23 - 15:26 ----

> [Noob here, sorry if I'm doing anything wrong.]
> 
> What is the minimum size of a simple single-disk Btrfs volume,
> and where is it documented ?  I can't find that info.
> 
> I'm told the minimum size is about 109 MB (114294784 bytes).
> True ?  Is there any way to get around that, at mkfs-time ?
> I'd like to use Btrfs inside a VeraCrypt container, and that's
> a fairly big minimum size for that use.
> 
> I'm using Btrfs on Ubuntu GNOME 20.04 desktop.
> 
> Thanks,
> 
> Bill Dietrich
> bill@billdietrich.me
> 
> 
> 
> -- Email domain proudly hosted at https://migadu.com

I'm not sure what minium limit is, but you can try using mkfs.btrfs --mixed. This worked with a 50MiB fs on btrfs-progs 5.7:

# btrfs fi us /mnt/loop
Overall:
    Device size:		  50.00MiB
    Device allocated:		   9.00MiB
    Device unallocated:		  41.00MiB
    Device missing:		     0.00B
    Used:			  32.00KiB
    Free (estimated):		  45.16MiB	(min: 45.16MiB)
    Data ratio:			      1.00
    Metadata ratio:		      1.00
    Global reserve:		 832.00KiB	(used: 0.00B)
    Multiple profiles:		        no

Data+Metadata,single: Size:5.00MiB, Used:28.00KiB (0.55%)
   /dev/loop0	   5.00MiB

System,single: Size:4.00MiB, Used:4.00KiB (0.10%)
   /dev/loop0	   4.00MiB

Unallocated:
   /dev/loop0	  41.00MiB

Re: Minimum size of Btrfs volume ?

[A L]

On 2020-08-23 17:08, A L wrote:
>
> ---- From: Bill Dietrich <bill@billdietrich.me> -- Sent: 2020-08-23 - 15:26 ----
>
>> [Noob here, sorry if I'm doing anything wrong.]
>>
>> What is the minimum size of a simple single-disk Btrfs volume,
>> and where is it documented ?  I can't find that info.
>>
>> I'm told the minimum size is about 109 MB (114294784 bytes).
>> True ?  Is there any way to get around that, at mkfs-time ?
>> I'd like to use Btrfs inside a VeraCrypt container, and that's
>> a fairly big minimum size for that use.
>>
>> I'm using Btrfs on Ubuntu GNOME 20.04 desktop.
>>
>> Thanks,
>>
>> Bill Dietrich
>> bill@billdietrich.me
>>
>>
>>
>> -- Email domain proudly hosted at https://migadu.com
> I'm not sure what minium limit is, but you can try using mkfs.btrfs --mixed. This worked with a 50MiB fs on btrfs-progs 5.7:
>
> # btrfs fi us /mnt/loop
> Overall:
>      Device size:		  50.00MiB
>      Device allocated:		   9.00MiB
>      Device unallocated:		  41.00MiB
>      Device missing:		     0.00B
>      Used:			  32.00KiB
>      Free (estimated):		  45.16MiB	(min: 45.16MiB)
>      Data ratio:			      1.00
>      Metadata ratio:		      1.00
>      Global reserve:		 832.00KiB	(used: 0.00B)
>      Multiple profiles:		        no
>
> Data+Metadata,single: Size:5.00MiB, Used:28.00KiB (0.55%)
>     /dev/loop0	   5.00MiB
>
> System,single: Size:4.00MiB, Used:4.00KiB (0.10%)
>     /dev/loop0	   4.00MiB
>
> Unallocated:
>     /dev/loop0	  41.00MiB
>
I was curious, so I went lower. Seems the limit is 16MiB using --mixed:

  # fallocate -l 10MiB loop.img
  # mkfs.btrfs -M loop.img
btrfs-progs v5.7
See http://btrfs.wiki.kernel.org for more information.

ERROR: 'loop.img' is too small to make a usable filesystem
ERROR: minimum size for each btrfs device is 16777216

Re: Minimum size of Btrfs volume ?

[Chris Murphy]

On Sun, Aug 23, 2020 at 7:26 AM Bill Dietrich <bill@billdietrich.me> wrote:
>
> [Noob here, sorry if I'm doing anything wrong.]
>
> What is the minimum size of a simple single-disk Btrfs volume,
> and where is it documented ?  I can't find that info.
>
> I'm told the minimum size is about 109 MB (114294784 bytes).
> True ?  Is there any way to get around that, at mkfs-time ?
> I'd like to use Btrfs inside a VeraCrypt container, and that's
> a fairly big minimum size for that use.
>

If this is backed by a loop mounted file on a file system supporting
sparse files:

1. make the backing file a bit bigger than you need, e.g. truncate -s
1g filename
2. cryptsetup open --allow-discards so that trim commands passthrough
dm-crypt to the backing file
3. fstrim /mountpoint - this "shrinks" the backing file by making it a
sparse file, file system size is unchanged
4. If the backing file needs to go on a file system that doesn't
support sparse files, use 'tar --sparse'

-- 
Chris Murphy