Re: Bug in btrfs-debug-tree for two or more devices.

[Hugo Mills <hugo@carfax.org.uk>]

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On Tue, Jun 12, 2012 at 06:53:00AM +0000, Santosh Hosamani wrote:
> 
> Hi btrfs folks,
>                 I am working on btrfs filesystem on how it manages the free space. And found out btrfs maintain a ctree which manages the physical location of the chunks and stripes of the filesystem.
> Btrfs-debug-tree also gives the information on the chunk tree
> 
> I created btrfs on single device and two device.I have attached the output of both on running btrfs-debug-tree.
> For single device sum of all the length in the chunks will add upto the total used bytes which is expected behavior.
> 
> But for two devices sum of all lengths in the chunks does not add to the total bytes .Am I missing something .

   Without actually seeing the details of your technique and
expectations, I shall make a guess that you're not accounting for the
double-counting of RAID-1 metadata. In other words, you will find that
all of the metadata device extents (or chunks) will appear twice --
once on each device.

   Actually, this isn't quite right either -- what you really need to
do is look at the RAID-1, RAID-10 and DUP bits in the chunk flags, add
up all of those chunks, divide by two, and then add in the remaining
(RAID-0 and single) chunks. That total should then add up to the total
value of allocated space that you get from the output of "btrfs fi df".

> Also I notice that for the second device the superblock location 0x10000 is not considered as used .
> 
> I would be really grateful if you folks can answer my query.
> 
> I hav run these tests on SLES11-sp2-x86
> Kernel 3.0.13.0.27-default

   This is pretty old, but shouldn't affect the results. It will cause
reliability problems if you try running it seriously.

   Hugo.

-- 
=== Hugo Mills: hugo@... carfax.org.uk | darksatanic.net | lug.org.uk ===
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