[linux-lvm] Possible bug in expanding thinpool: lvextend doens't expand the top-level dm-linear device

M.H. Tsai mingnus at gmail.com
Fri Jan 1 18:10:45 UTC 2016

2016-01-01 5:25 GMT+08:00 Zdenek Kabelac <zkabelac at redhat.com>:
> You should be aware of  thin-pool limits.
> i.e. ATM it's bad plan to use more then say 16 LVs in parallel for
> a single thin-pool LV - it cannot be used in some massive parallel system
> for its current locking mechanism.

Is it LVM or dm-thin kernel target's limit? And, is there any
reference about the "16 LVs" and the locking issue? (why 16 LVs?)

> There is even sequencing problem with creating snapshot in kernel target
> which needs to be probably fixed first.
> (the rule here should be - to never create/allocate something when
> there is suspended device - and this rule is broken with current thin
> snapshot creation - so thin snap create message should go in front
> to ensure there is a space in thin-pool ahead of origin suspend  - will
> be addressed in some future version....)
> However when taking snapshot - only origin thin LV is now suspended and
> should not influence rest of thin volumes (except for thin-pool commit
> points)

Does that mean in future version of dm-thin, the command sequence of
snapshot creation will be:

dmsetup message /dev/mapper/pool 0 "create_snap 1 0"
dmsetup suspend /dev/mapper/thin
dmsetup resume /dev/mapper/thin

> minor warning - snapshot is not a backup - although it might look like it is
> :).

Yes, we know it, thanks :)

Ming-Hung Tsai

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