XFS AG disk layout described on official doc is wrong

XFS AG disk layout described on official doc is wrong

[Jeff Liu]

Hello,

I observed that there is an issue on our official DOC for AG disk
layout, which can be found at:
http://xfs.org/docs/xfsdocs-xml-dev/XFS_Filesystem_Structure//tmp/en-US/html/Allocation_Groups.html

It shows that the IBT block is followed by the block where AGFL sector
resides.  However, it should be followed by the
CNT block according to my debug, like below:

-------------------------------
superblock
AG free block info
AG inode B+tree info
AG internal free list
-------------------------------
Root of free space B+tree
  B+tree key is block number
-------------------------------
Root of free space B+tree
  B+tree key is block count
--------------------------------
Root of inodes B+tree
--------------------------------

Maybe the DOC is old, or am I missing anything?  If not, can we correct it?


Thanks,
-Jeff

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Re: XFS AG disk layout described on official doc is wrong

[Dave Chinner]

On Fri, Nov 09, 2012 at 02:47:41PM +0800, Jeff Liu wrote:
> Hello,
> 
> I observed that there is an issue on our official DOC for AG disk
> layout, which can be found at:
> http://xfs.org/docs/xfsdocs-xml-dev/XFS_Filesystem_Structure//tmp/en-US/html/Allocation_Groups.html
> 
> It shows that the IBT block is followed by the block where AGFL sector
> resides.  However, it should be followed by the
> CNT block according to my debug, like below:
> 
> -------------------------------
> superblock
> AG free block info
> AG inode B+tree info
> AG internal free list
> -------------------------------
> Root of free space B+tree
>   B+tree key is block number
> -------------------------------
> Root of free space B+tree
>   B+tree key is block count
> --------------------------------
> Root of inodes B+tree
> --------------------------------
> 
> Maybe the DOC is old, or am I missing anything?  If not, can we correct it?

The btree root blocks are dynamic. mkfs.xfs puts them in a fixed
place, just like growfs does, but the moment you split a root block,
it moves from that location. Hence the only way to reliably find the
root btree block is to look up the point in the AGF/AGI, and that's
what the document shows. IOWs, the ordering in the document is not
important - the linkage is - as the root blocks can end up
*anywhere* in the AG.

e.g:

$ sudo xfs_db -r -c "agf 0" -c p /dev/vda
magicnum = 0x58414746
versionnum = 1
seqno = 0
length = 262144
bnoroot = 596
cntroot = 1003
bnolevel = 2
cntlevel = 2
....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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Re: XFS AG disk layout described on official doc is wrong

[Jeff Liu]

On 11/09/2012 03:36 PM, Dave Chinner wrote:
> On Fri, Nov 09, 2012 at 02:47:41PM +0800, Jeff Liu wrote:
>> Hello,
>>
>> I observed that there is an issue on our official DOC for AG disk
>> layout, which can be found at:
>> http://xfs.org/docs/xfsdocs-xml-dev/XFS_Filesystem_Structure//tmp/en-US/html/Allocation_Groups.html
>>
>> It shows that the IBT block is followed by the block where AGFL sector
>> resides.  However, it should be followed by the
>> CNT block according to my debug, like below:
>>
>> -------------------------------
>> superblock
>> AG free block info
>> AG inode B+tree info
>> AG internal free list
>> -------------------------------
>> Root of free space B+tree
>>   B+tree key is block number
>> -------------------------------
>> Root of free space B+tree
>>   B+tree key is block count
>> --------------------------------
>> Root of inodes B+tree
>> --------------------------------
>>
>> Maybe the DOC is old, or am I missing anything?  If not, can we correct it?
> 
> The btree root blocks are dynamic. mkfs.xfs puts them in a fixed
> place, just like growfs does, but the moment you split a root block,
> it moves from that location. Hence the only way to reliably find the
> root btree block is to look up the point in the AGF/AGI, and that's
> what the document shows. IOWs, the ordering in the document is not
> important - the linkage is - as the root blocks can end up
> *anywhere* in the AG.
> 
> e.g:
> 
> $ sudo xfs_db -r -c "agf 0" -c p /dev/vda
> magicnum = 0x58414746
> versionnum = 1
> seqno = 0
> length = 262144
> bnoroot = 596
> cntroot = 1003
> bnolevel = 2
> cntlevel = 2
> ....
Ah, thanks for your quick response, now I understood. :)

Regards,
-Jeff

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