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:
> 
> 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:

OK.  So with an empty filesystem, before my first copy (i.e. the base on
which the next copy will CoW from) df reports:

Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/mapper/btrfs--test-btrfs--test
                     922746880        56 922746824   1% /mnt/btrfs-test

and btrfs fi df reports:

Data: total=8.00MB, used=0.00
Metadata: total=1.01GB, used=24.00KB
System: total=12.00MB, used=4.00KB

after the first copy df and btrfs fi df report:

Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/mapper/btrfs--test-btrfs--test
                     922746880 121402328 801344552  14% /mnt/btrfs-test

root@linux:/mnt/btrfs-test# cat .snapshots/monthly.22/metadata/btrfs_df-stop
Data: total=110.01GB, used=109.26GB
Metadata: total=5.01GB, used=3.26GB
System: total=12.00MB, used=24.00KB

So it's clear that total usage (as reported by df) was 121,402,328KB but
Metadata has two values:

Metadata: total=5.01GB, used=3.26GB

What's the difference between total and used?  And for that matter,
what's the difference between the total and used for Data
(total=110.01GB, used=109.26GB)?

Even if I take the largest values (i.e. the total values) for Data and
Metadata (each converted to KB first) and add them up they are:
120,607,211.52 which is not quite the 121,402,328 that df reports.
There is a 795,116.48KB discrepancy.

In any case, which value from a btrfs df fi should I be subtracting from
df's accounting to get a real accounting of the amount of data used?

Cheers,
b.


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