Re: efficiency of btrfs cow

["Brian J. Murrell" <brian@interlinx.bc.ca>]

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On 11-03-06 11:06 AM, Calvin Walton wrote:
> 
> There actually is such a periodic jump in overhead,

Ahh.  So my instincts were correct.

> caused by the way
> which btrfs dynamically allocates space for metadata as needed by the
> creation of new files, which it does whenever the free metadata space
> ratio reaches a threshold (it's probably more complicated than that, but
> close enough for now).

Sounds fair enough.

> To see exactly what's going on, you should use the "btrfs filesystem df"
> command to see how space is being allocated for data and metadata
> separately:
> 
> ayu ~ # btrfs fi df /
> Data: total=266.01GB, used=249.35GB
> System, DUP: total=8.00MB, used=36.00KB
> Metadata, DUP: total=3.62GB, used=1.93GB
> ayu ~ # df -h /
> Filesystem            Size  Used Avail Use% Mounted on
> /dev/sda4             402G  254G  145G  64% /
> 
> If you use the btrfs tool's df command to account for space in your
> testing, you should get much more accurate results.

Indeed!  Unfortunately that tool seems to be completely silent on my system:

# btrfs filesystem df /mnt/btrfs-test/
# btrfs filesystem df /mnt/btrfs-test

Where /mnt/btrfs-test is where I have the device that I created the
btrfs filesystem on mounted.  i.e.:

# grep btrfs /proc/mounts
/dev/mapper/btrfs--test-btrfs--test /mnt/btrfs-test btrfs rw,relatime 0 0

My btrfs-tools appears to be from 20101101.  The changelog says:

  * Merging upstream version 0.19+20101101.

Cheers,
b.


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