Liu Bo wrote on 2015/11/14 22:35 -0800:
Hi,
On Fri, Nov 13, 2015 at 09:41:01AM -0800, Marc MERLIN wrote:
root@polgara:/mnt/btrfs_root# du -sh *
28G @
28G @_hourly.20151113_08:04:01
4.0K @_last
4.0K @_last_rw
28G @_rw.20151113_00:02:01
root@polgara:/mnt/btrfs_root# df -h .
Filesystem Size Used Avail Use% Mounted on
/dev/sdb5 56G 40G 5.4G 89% /mnt/btrfs_root
root@polgara:/mnt/btrfs_root# btrfs fi df .
Data, single: total=39.85GiB, used=38.52GiB
System, DUP: total=8.00MiB, used=16.00KiB
System, single: total=4.00MiB, used=0.00B
Metadata, DUP: total=6.00GiB, used=579.17MiB
Metadata, single: total=8.00MiB, used=0.00B
GlobalReserve, single: total=208.00MiB, used=0.00B
root@polgara:/mnt/btrfs_root# btrfs fi show .
Label: 'btrfs_root' uuid: a2a1ed7b-6bfe-4e83-bc10-727126ed17bf
Total devices 1 FS bytes used 39.09GiB
devid 1 size 55.88GiB used 51.88GiB path /dev/sdb5
btrfs-progs v4.0-dirty
root@polgara:/mnt/btrfs_root#
root@polgara:/mnt/btrfs_root# btrfs balance start -dusage=80 -v /mnt/btrfs_root
Dumping filters: flags 0x1, state 0x0, force is off
DATA (flags 0x2): balancing, usage=80
Done, had to relocate 1 out of 55 chunks
Sadly, it's only running 3.17.8 because of complicated reasons, but still,
1) I have 28GB used (modulo a few files between the btrfs send snapshots and
current status)
2) fi show shows I'm using 39GB, not sure where the extra 11GB came from
3) fi df agrees with fi show
4) regular df agrees on used too, but shows 5GB free instead of 15GB despite
the filesystem being balanced.
I did have a bunch of snapshots that I did delete a while ago now, but it
looks like their blocks aren't being reclaimed.
Any ideas?
Since you said you have some snapshots in between...I can think of one
case to prove where the space goes,
Say, you have a file with size=10M on a freshly created partition(the total used data space is 10M), and you have a snapshot which owns this file, then you modify the original file by overwrite the range [3M, 5M], and right now you can find that the total used data space increases to 15M or maybe more (because of unaliged write and extent pads to 4K length).
This comes from our COW and extent references implementation, so you get
the benefit of COW, meanwhile have to live with the un-reclaimed space.
It's sort of something I was trying to fix, but I found that my approach
led to other problems so I decided to give it up.
Thanks,
-liubo
The case is quite right, but the example maybe a little incorrect BTW.
For 10M file in one subvolume and rewrite [3M, 5M](including the last
byte) in the snapshot will only increase used space to 12M + 4K, as
above numbers except the last byte are already sectorsize aligned.
The root cause is, btrfs' lazy extent freeing behavior.
Use a new 12M case to describe:
======
Step 0:
Subv A:
[0,12M): Shared between A and B (Extent 1)
on disk:
Extent 1: record the above [0,12M) data
Subv B:
[0,4M): Shared between A and B (Extent 1)
[4M,8M): Exclusive in B (Extent 2)
[8M,12M): Shared between A and B (Extent 1)
on disk:
Extent 1: is reused in [0,4M) and [8M,12M) ranges
Extent 2: record data of [4M,8M)
------
Step 1:
Write [4M,8M) of subv A:
[0,4M): Shared between A and B (Extent 1)
[4M,8M): Exclusive in A (Extent 3)
[8M,12M): Shared between A and B (Extent 1)
Extent 3: record the above [4M,8M) data
And Extent 1 is not changed.
Subv B:
[0,4M): Shared between A and B (Extent 1)
[4M,8M): Exclusive in B (Extent 2)
[8M,12M): Shared between A and B (Extent 1)
======
After step 2, the used space will be 12M (Extent 1) + 4M (Extent 2) + 4M
(Extent 3) = 20M.
Even only 8M of extent 1 is really referred by.
So the middle 4M of extent 1 is totally wasted and Btrfs won't free it,
until *ALL* of the extent is not referred by any one.
One solution would be defrag, but IIRC defrag under multi-subvolume case
is not supported yet...
BTW, personally speaking, to find out how much space a subvolume takes,
btrfs qgroup would be quite handy (after 4.2 kernel).
Shows how much space a subvolume takes exclusively.
Thanks,
Qu
Thanks,
Marc
--
"A mouse is a device used to point at the xterm you want to type in" - A.S.R.
Microsoft is to operating systems ....
.... what McDonalds is to gourmet cooking
Home page: http://marc.merlins.org/
--
To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
--
To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
--
To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
