[linux-lvm] inconsistency between thin pool metadata mapped_blocks and lvs output

[Joe Thornber]

On Thu, May 10, 2018 at 07:30:09PM +0000, John Hamilton wrote:
> I saw something today that I don't understand and I'm hoping somebody can
> help.  We had a ~2.5TB thin pool that was showing 69% data utilization in
> lvs:
> 
> # lvs -a
>   LV                    VG       Attr       LSize  Pool Origin Data%
> Meta%  Move Log Cpy%Sync Convert
>   my-pool         myvg twi-aotz--  2.44t             69.04  4.90
>   [my-pool_tdata] myvg Twi-ao----  2.44t
>   [my-pool_tmeta] myvg ewi-ao---- 15.81g
> 
> However, when I dump the thin pool metadata and look at the mapped_blocks
> for the 2 devices in the pool, I can only account for about 950GB.  Here is
> the superblock and device entries from the metadata xml.  There are no
> other devices listed in the metadata:
> 
> <superblock uuid="" time="34" transaction="68" flags="0" version="2"
> data_block_size="128" nr_data_blocks="0">
>   <device dev_id="1" mapped_blocks="258767" transaction="0"
> creation_time="0" snap_time="14">
>   <device dev_id="8" mapped_blocks="15616093" transaction="27"
> creation_time="15" snap_time="34">
> 
> That first device looks like it has about 16GB allocated to it and the
> second device about 950GB.  So, I would expect lvs to show somewhere
> between 950G-966G Is something wrong, or am I misunderstanding how to read
> the metadata dump?  Where is the other 700 or so GB that lvs is showing
> used?

The non zero snap_time suggests that you're using snapshots.  I which case it
could just be there is common data shared between volumes that is getting counted
more than once.

You can confirm this using the thin_ls tool and specifying a format line that
includes EXCLUSIVE_BLOCKS, or SHARED_BLOCKS.  Lvm doesn't take shared blocks into
account because it has to scan all the metadata to calculate what's shared.

- Joe