[dm-devel] [PATCH 2/3] dm thin metadata: do not write metadata if no changes occurred
Mike Snitzer
snitzer at redhat.com
Tue Apr 16 02:39:03 UTC 2019
On Mon, Apr 15 2019 at 5:17pm -0400,
Mike Snitzer <snitzer at redhat.com> wrote:
> Otherwise, just loading a thin-pool and then removing it will cause its
> metadata to be changed (e.g. superblock written) -- even without any
> thin devices being made active or metadata snapshot being manipulated.
>
> Add 'changed' flag to struct dm_pool_metadata. Set both td->changed and
> pmd->changed using __set_thin_changed() -- this assures pmd->changed is
> always set when td->changed is. For methods that don't involve changing
> a td, set pmd->changed directly.
>
> Rename __delete_device() to __delete_thin() -- improves symmetry with
> __create_thin().
>
> Update dm_pool_changed_this_transaction() to simply return pmd->changed.
>
> Fix dm_pool_commit_metadata() to open the next transaction if the return
> from __commit_transaction() is 0. Not seeing why the early return ever
> made since for a return of 0 given that dm-io's async_io(), as used by
> bufio, always returns 0.
This v1 patch was incomplete. However this approach's need to establish
pmd->changed everywhere that alters metadata, without doing so in terms
of a thin device, is like whack-a-mole.
But I think this could be simplified to only set a 'pmd->accessed'
(never clear it) and only__commit_transaction() if pmd->accessed --
so only pool messages that change metadata would need to set
pmd->accessed.
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