Question about AGFL

Question about AGFL

[Sergey Stepanov]

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Hi.

I have a device with XFS on it and if I run xfs_db on it, I get the
following:

xfs_db> agf 0
xfs_db> p
magicnum = 0x58414746
versionnum = 1
seqno = 0
length = 6553344
bnoroot = 19432
cntroot = 20689
bnolevel = 1
cntlevel = 1
flfirst = 0
fllast = 3
flcount = 4
freeblks = 6529632
longest = 6522442
btreeblks = 0
xfs_db> agfl 0
xfs_db> p
bno[0-127] = 0:1 1:2 2:3 3:4

As you can see, AGFL contains block numbers 1-4.

As far as I understand, blocks 1-3 contain roots for inode and free space
trees, so it seems really suspicious to me that they are listed in free
list.
I  digged a bit into XFS code inside Linux kernel and did not find any code
that will prevent overwriting them when B+trees will start to grow.

So, the question is did I miss something and this is a normal situation, or
is this some kind of bug or corruption?

Thanks in advance.

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Re: Question about AGFL

[Dave Chinner]

On Thu, Oct 18, 2012 at 07:27:09PM +0400, Sergey Stepanov wrote:
> Hi.
> 
> I have a device with XFS on it and if I run xfs_db on it, I get the
> following:
> 
> xfs_db> agf 0
> xfs_db> p
> magicnum = 0x58414746
> versionnum = 1
> seqno = 0
> length = 6553344
> bnoroot = 19432
> cntroot = 20689
> bnolevel = 1
> cntlevel = 1
> flfirst = 0
> fllast = 3
> flcount = 4
> freeblks = 6529632
> longest = 6522442
> btreeblks = 0
> xfs_db> agfl 0
> xfs_db> p
> bno[0-127] = 0:1 1:2 2:3 3:4
> 
> As you can see, AGFL contains block numbers 1-4.
> 
> As far as I understand, blocks 1-3 contain roots for inode and free space
> trees, so it seems really suspicious to me that they are listed in free
> list.

Roots of the freespace btrees are at blocks:

> bnoroot = 19432
> cntroot = 20689

Root blocks move around, because the are reallocated during splits
change during merges.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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