Hi Alex,
On 11/29/2013 01:39 AM, Alex Lyakas wrote:
Hello Anand,
I have sent a similar comment to your email thread in
http://www.spinics.net/lists/linux-btrfs/msg27547.html
I believe this approach of calculating freeable space is incorrect.
Try this:
- create a fresh btrfs
- create a regular file
- write some data into it in such a way, that it was, say 4000
EXTENT_DATA items, so that file tree and extent tree get deep enough
- run btrfs-debug-tree and verify that all EXTENT_ITEMs of this file
(in the extent tree) have refcnt=1
- create a snapshot of the subvolume
- run btrfs-debug-tree again
You will see that most (in my case - all) of EXTENT_ITEMs still have
refcnt=1. Reason for this is as I mentioned in
http://www.spinics.net/lists/linux-btrfs/msg27547.html
But if you delete the subvolume, no space will be freed, because all
these extents are also shared by the snapshot. Although it seems that
your tool will report that all subvolume's space is freeable (based on
refcnt=1).
Can you pls try that experiment and comment on it? Perhaps I am
missing something here?
I think you are right here, the only way we can make sure whether
a extent_item is shared by many roots is to walk backref to find
all roots, and if number of roots is 1, we know it's sole space.
If so, i don't think this should be implemented in user space, Btrfs quota
implement such function in kernel space, but it also face a problem that
deleting a subvolume will break space accounting(because subvolume deletion
won't iterate the whole fs tree).
Thanks,
Wang
Thanks!
Alex.
On Thu, Oct 10, 2013 at 6:33 AM, Wang Shilong
<wangsl.f...@cn.fujitsu.com> wrote:
On 10/10/2013 11:35 AM, Anand Jain wrote:
If 'btrfs_file_extent_item' can contain the ref count it would
solve the current problem quite easily. (problem is that, its
of n * n searches to know data extents with its ref for a given
subvol).
Just considering btrfs_file_extent_item is not enough, because
we should consider metadata(as i have said before).
But what'r the challenge(s) to have ref count in the
btrfs_file_extent_item ? any thoughts ?
Haven't thought a better idea yet.
Thanks, Anand
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