Hi,

A good rough estimate would be the total of the space
that is displayed under the "USED" column of "zfs list" for those snapshots.

Here is an example :
-- snip --
[EMAIL PROTECTED] zfs list -r tank
NAME         USED  AVAIL  REFER  MOUNTPOINT
tank        24.6M  38.9M    19K  /tank
tank/fs1    24.4M  38.9M    18K  /tank/fs1
tank/[EMAIL PROTECTED]  24.4M      -  24.4M  -
-- snip --

In the above case tank/[EMAIL PROTECTED] is using 24.4M. So, if we delete
that snapshot it would freeup about 24.4M. Let's delete it an
see what we get :

-- snip --
[EMAIL PROTECTED] zfs destroy tank/[EMAIL PROTECTED]
[EMAIL PROTECTED] zfs list -r tank
NAME       USED  AVAIL  REFER  MOUNTPOINT
tank       220K  63.3M    19K  /tank
tank/fs1    18K  63.3M    18K  /tank/fs1
-- snip --

So, we did get back 24.4M freed (39.9M + 24.4M = 63.3M).

Note that this could get a little complicated if there are multiple
snapshots which refer to the same set of blocks. So, even after deleting
one snapshot you might not see the space freed up. And this could be because,
of the second snapshot which is refering to some of the blocks still.

Hope that helps.

Thanks and regards,
Sanjeev
 
On Wed, Dec 03, 2008 at 12:26:48AM +0000, Robert Milkowski wrote:
> Hello none,
> 
> Thursday, November 6, 2008, 7:55:42 PM, you wrote:
> 
> n> Hi Milek,
> n> Thanks for your reply.
> n> What I really need is a way to tell how much space will be freed
> n> for any particular set of snapshots that I delete. 
> 
> n> So I would like to query zfs,
> n> "if I delete these snapshots
> n> storage/[EMAIL PROTECTED] 
> n> storage/[EMAIL PROTECTED]
> n> how much space will be freed?" 
> 
> I'm afraid you can do only one at a time.
> 
> -- 
> Best regards,
>  Robert                            mailto:[EMAIL PROTECTED]
>                                        http://milek.blogspot.com
> 
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