I'm not a developer, but I think it goes something like this: btrfs doesn't write the filesystem on the entire device/partition at format time, rather, it dynamically increases the size of the filesystem as data is used. That's why formating a disk in btrfs can be so fast.
On Wed, Mar 23, 2011 at 12:39 PM, Brian J. Murrell <br...@interlinx.bc.ca> wrote: > > 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. > -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
