On Oct 25, 2012, at 1:21 PM, Goffredo Baroncelli <kreij...@inwind.it> wrote:
>
> Moreover I still didn't understand how btrfs was using the disks.
This comment has less to do with the RFC, and more about user confusion in a
specific case of the existing fi df behavior. But since I have the same
misunderstanding of how btrfs is using the disks, I decided to reply to this
thread.
While working with Fedora 18's new System Storage Manager [1], I came across
this problem. For reference the bug report [2] which seems less of a bug with
ssm than a peculiarity with btrfs chunk allocation and how fi df report usage.
80GB VDI, Virtual Box VM, containing Fedora 18: installed and yum updated 2-3
times. That's it, yet for some reason, 76 GB of chunks have been allocated and
they're all full? This doesn't make sense when there's just under 4GB of data
on this single device.
[root@f18v ~]# btrfs fi show
Label: 'fedora' uuid: 780b8553-4097-4136-92a4-c6fd48779b0c
Total devices 1 FS bytes used 3.93GB
devid 1 size 76.06GB used 76.06GB path /dev/sda1
[root@f18v ~]# btrfs fi df /
Data: total=72.03GB, used=3.67GB
System, DUP: total=8.00MB, used=16.00KB
System: total=4.00MB, used=0.00
Metadata, DUP: total=2.00GB, used=257.54MB
Metadata: total=8.00MB, used=0.00
I decided to rebalance, and while things become a lot more sensible, I'm still
confused:
[chris@f18v ~]$ sudo btrfs fi show
failed to read /dev/sr0
Label: 'fedora' uuid: 780b8553-4097-4136-92a4-c6fd48779b0c
Total devices 1 FS bytes used 3.91GB
devid 1 size 76.06GB used 9.13GB path /dev/sda1
[chris@f18v ~]$ sudo btrfs fi df /
Data: total=5.00GB, used=3.66GB
System, DUP: total=64.00MB, used=4.00KB
System: total=4.00MB, used=0.00
Metadata, DUP: total=2.00GB, used=256.84MB
Points of confusion:
1. Why is FS bytes used = 3.91GB, yet devid 1 used is 9.13 GB?
2. Why before a rebalance does 'fi df' show extra lines, and then after
rebalance there are fewer lines? Another case with raid10, 'fi df' shows six
lines of data, but then after rebalance is shows three lines?
3. How does Data: total=72GB before rebalance, but is 5GB after rebalance? This
was a brand new file system, file system installed, with maybe 2-3 updates, and
a dozen or two reboots. That's it. No VM's created on that volume (it's a VDI
itself), and the VDI file itself never grew beyond 9GB.
Chris Murphy
[1] https://fedoraproject.org/wiki/Features/SystemStorageManager
[2] https://bugzilla.redhat.com/show_bug.cgi?id=869489--
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