David Sterba wrote on 2015/09/09 15:25 +0200:
On Wed, Sep 09, 2015 at 11:24:17AM +0800, Qu Wenruo wrote:
The problem is that, for converted image, it's quite possible that data
and metadata extent are stored in one chunk even the chunk is not mixed.
I'll add fsck support for it soon.
That would be great, I have no idea how the data/metadata can get mixed.
Does this involve inlied file data that come from ext4?
Not completely sure yet, but in my case, I didn't put any data into the
ext4 fs, just newly created one and convert.
And no inline data extent in converted btrfs, checked by debug-tree.
So I think the problem is the chunk allocation algorithm for btrfs-convert.
The algorithm just doesn't check whether the allocated chunk will cover
ext* data or not, and determine chunk type by a strange data/metadata
threshold.
To fix it, we'd better rework the algorithm to add more flexibility to
handle scattered ext* data.
But at the bottom line, every ext* used space should be covered by DATA
chunk.
Like the following case:
0 1G 2G 3G
|//| |///| |//| |//|
Here For range 0~1G, the beginning is some ext* data, but doesn't take
too much space.
And in range 1G~2G, some ext* extents are scattered there.
Lastly 2G~3G is not used.
For 0~1G, we only need to ensure the beginning part(including the ext*
range) be covered by a data chunk, the resting part can be either
allocated for metadata or data of newer btrfs chunk.
For 1G~2G, better to allocate the whole 1G into data chunk, as too many
scattered ext* space.
For 2G~3G, either DATA or METADATA is OK for further allocation.
But for now, as a hot fix, I prefer the quick but dirty fix that I
already submitted, just to force btrfs-convert to use mixed-bg patchset
(with fsck support).
The roadmap for convert fix will follow then:
1) Add kernel balance support to convert mixed-bg.
Seems easier than 2) for me.
2) Add btrfs-convert support to use correct non mixed-bg mode.
But I'm still not sure about whether the bug mail list reported is
related to the bug we exposed.
Thanks,
Qu
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