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question about one example in section 3.2.3 of "XFS Filesystem Structure

To: "xfs" <xfs@xxxxxxxxxxx>
Subject: question about one example in section 3.2.3 of "XFS Filesystem Structure"
From: "Wang Sheng-Hui" <shhuiw@xxxxxxxxxxx>
Date: Thu, 29 Jan 2015 18:39:51 +0800
Cc: "david" <david@xxxxxxxxxxxxx>
Delivered-to: xfs@xxxxxxxxxxx
Dear,

I'm reading "XFS Filesystem Structure" from the link 
             
http://xfs.org/docs/xfsdocs-xml-dev/XFS_Filesystem_Structure//tmp/en-US/html/index.html
and confused by one example in section 3.2.3:
             
http://xfs.org/docs/xfsdocs-xml-dev/XFS_Filesystem_Structure//tmp/en-US/html/AG_Free_List.html
---------------------------------------------------------------------------
xfs_db> fsblock 7
xfs_db> type bnobt
xfs_db> p
magic = 0x41425442
level = 1
numrecs = 4
leftsib = null
rightsib = null
keys[1-4] = [startblock,blockcount]
           1:[12,16] 2:[184586,3] 3:[225579,1] 4:[511629,1]
ptrs[1-4] = 1:2 2:83347 3:6 4:4

Blocks 2, 83347, 6 and 4 contain the leaves for the free space B+tree by 
starting block. Block 2 would contain offsets 16 up to but not including 184586 
while block 4 would have all offsets from 511629 to the end of the AG. 
---------------------------------------------------------------------------

I have thought block 2 contains offsets start from 12, and it will cover 16 
free blocks.
I wonder why block 2 contains offsets start at 16, instead of 12 here?

Anyone help give out some explanation, please?

Regards,
Wang Sheng-Hui
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