[illumos-Developer] Review --> iSCSI UNMAP support

[Albert Lee]

On Thu, Feb 10, 2011 at 12:10 AM, Dan McDonald <danmcd at nexenta.com> wrote:
> On Wed, Feb 09, 2011 at 10:07:14PM -0500, Eric Schrock wrote:
>> Can you explain why unmap is only supported for zvols without a
>> refreservation?
>
> No, I can't.  I took this existing code and dropped it in.  I took no part in
> its design.  You know my background isn't in ZFS, so I'm willing to accept
> any clues here, even if I can't act upon them immediately.
>
>> Freeing this data
>
> You mean dmu_free_long_range(), right?
>
>> can have meaningful impact on future operations (for example, subsequent
>> snapshots, or changing the zvol to be sparse).  Seems like extra work and
>> causes more harm than good.
>
> Interesting.  I believe the code forces the zvol to a minimum of zle
> compression (see the do_unmap_setup() function).  Wouldn't this allow the
> dmu_free_long_range() to be at least space-efficient?

Is zle really necessary for the freed region to be reused by other
objects? Should we check for SPA_VERSION_ZLE_COMPRESSION?

-Albert