[dm-devel] thinp zeroing
Joe Thornber
thornber at redhat.com
Thu Feb 23 15:47:19 UTC 2012
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 at 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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