-------- Original Message --------
Subject: Re: About leaf corruption recovery(currently only fs/subvol
tree recovery)
From: Josef Bacik <jba...@fb.com>
To: Qu Wenruo <quwen...@cn.fujitsu.com>, linux-btrfs
<linux-btrfs@vger.kernel.org>
Date: 2014年11月13日 22:43
On 11/13/2014 04:02 AM, Qu Wenruo wrote:
Hi all,
I'm trying to implement leaf corruption recovery.
*CURRENT BEHAVIOR*
Btrfs now heavily rely on chunk level duplication to protect its tree
block(meta data).
That's completely good and works quite well.
However small device with mixed single chunk will suffer from the lack
of duplication and when any
bit flip happens in tree block, the whole 16K leaf/node will be
unreadable and finally cause
metadata corruption.
*OBJECT*
I hope btrfsck can repair such bit flip even with the cost of data lose.
(It will of course introduce data loss according to the following
method)
And the ultimate object will be making a randomly slightly(0.2% of all
bytes?) damaged btrfs
can pass btrfsck after repair.
*RECOVERY METHOD*
Current recovery method is consist of the following procedure:
1) find and record the unreadable extent buffers during normal fsck
routine
With the record of the unreadable extent buffers, we can calculates the
inode number range where
next step will drop.
2) *delete* the slot pointing to the leaf in parent node
Yes, delete the corrupted leaves, at least this is the cleanest and
easiest method.
After the step, the metadata tree should at least be iteratable now.
3) cleanup the mess done in 2)
Need to do the following things in case btrfsck complains later
3.1) salvage data from extent tree in the deleting range.
Although fs/subvol leaf is deleted, extent data is still there, using
EXTENT_ITEM in extent tree
may still recover some data.
Personally I prefer to create a lost+found dir in the root of its
subvolume and use inode number as
file name to restore them.
3.2) Remove backref to the inodes in deleting ranges and move them if
needed.
It is clear we need to remove the invalid backref, but if some inodes in
deleting ranges casuing
its children files unaccessible from the subvolume root, then these
files should be moved to 'lost+found' too,
even they are completely undamaged.
Although after the above steps, metadata like filename, access bits,
owner, xattrs or inlined data will be
lost and some files/dirs will be moved to lost+found, it should at least
btrfsck not complain any more.
*NEED ADVICE*
Any concern about the above recovery is welcomed, especially when some
guy like me want to
implement such an aggressive recovery method.
So we already have a way to fix weird problems with blocks in btrfsck,
see try_to_fix_bad_block. This doesn't fix everything, but it could
easily be expanded to just add anybody who can't be fixed to a list to
be deleted and then see what fsck comes up with. If the block is in
the extent tree for example it's pretty easy to recover, fs tree's can
rebuild some missing stuff, csum tree doesn't do anything yet.
Great thanks for the hint on existing block fixing infrastructure.
I'll expand it.
I think the best bet is to track these bad blocks and then adjust what
we do based on which tree they are in.
Definitely, but currently I want to focus on the fs-tree parts, since
extent/csum/chunk tree can be somewhat rebuildable.
BTW, any comment about the drop-leaf-and-salvage-data idea for the
fs/subvolume tree recovery?
Thanks,
Qu
For example we don't want fsck just randomly re-generating data
csums, but if we've found a bad block in the csum tree then we
definitely want to re-generate the data csum in that case. But for
the extent tree we can be sure that we'll put stuff back in the right
way, so you can just remove that block and know the normal fsck code
will fix things. Thanks,
Josef
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