[dm-devel] Strange mapped block count kept by dm-thin driver for thin devices

[Zdenek Kabelac]

Dne 28.8.2013 05:52, Teng-Feng Yang napsal(a):
> Hi,
>
> I have already noticed that the mapped block count of thin devices
> shown by "dmsetup status" looked unreasonable to me for a couple of
> times in the last six months.
> Sometimes the mapped block count was zero, but sometimes it was a
> large number which was obviously larger than the total available
> blocks in thin-pool.
> So I dig into the source code, and I find some codes look suspicious to me.
>
> In dm_pool_metadata_close() in dm-thin-metadata.c, it will try to
> commit transaction for the last time before it destroys the
> pmd(dm_pool_metadata).
> However, it does not lock the pool_metadata before transaction commit,
> which I am not really sure if we still need to acquire lock when it
> reaches here.
> I know that this function is mainly called when users try to remove
> pool, but it seems like we can still have some on-the-fly transaction
> commit when dm-thin module calls this function.
> Although I would like to test it by myself, this problem occurs in
> random and I am still trying to find a way to reproduce it
> systematically.
>
> Any help would be grateful.
>

I think it's good idea in this case to catch as much info in the moment you 
think you see a problem.

Take  output from:

dmsetup status/info/table
safe content of your metadata device for further analysis with thin 
provisioning tools.

Are you using lvm2 for thin volumes ?

Zdenek