Re: [dm-devel] thinp zeroing

[Mike Snitzer <snitzer redhat com>]

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.

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