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Re: [dm-devel] thinp zeroing



On Wed, Feb 22, 2012 at 09:49:18PM -0500, Mike Snitzer wrote:
> Nice write-up.  It is concerning that we have to go to such lengths but
> I don't see a way around it without limiting who can consume thinp.
> 
> On Wed, Feb 22 2012 at  9:14am -0500,
> Joe Thornber <thornber redhat com> wrote:
> 
> > ** Discards
> > 
> >    DISCARDs *must* result in data being zeroed.  Some devices set the
> >    discard_zeroes_data flag.  This is not good enough; you cannot use
> >    this flag as a guarantee that the data no longer exists on the
> >    disk.  So real zeroing must occur.  I suggest we write a separate
> >    target that zeroes data just before discarding it, and stack it
> >    under the thin-pool.  The performance impact of this will be
> >    significant; to the point that we may wish to turn discard within
> >    the fs off; instead doing periodic tidy-ups.
> 
> ...
> 
> > ** Summary of work items [0/5]
> > 
> >    - [ ] Implement the discard-really-zeroes target [1 month]
> 
> I don't think it'll take a month.  Probably a focused week to 2 weeks.

By the time you include getting it through agk I think a month is
highly optimistic.

> I can develop this target before jumping in to the HSM target (unless
> you'd rather I start in on HSM asap).

HSM is the priority please.  ERASE can wait until later.  Plus given
the development effort and performance impact I think there are other
alternatives we should consider (such as using dm-crypt on each thin,
and throwing away the keys when you delete it).

- Joe


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