zfs discuss - Jul 2010 - How to calculate space consumption of some snapshots

Hello

Among all my snapshots, I''m interested about space consumption of some
of them. Is it possible to calculate how much space would be freed if  
I destroy only some of all snapshots?

Henrik Heino

----- Original Message -----
> Hello
> 
> Among all my snapshots, I''m interested about space consumption of
some
> of them. Is it possible to calculate how much space would be freed if
> I destroy only some of all snapshots?
zfs list -t snapshot

that''ll show you their sizes as well as other details
 
Vennlige hilsener / Best regards

roy
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>> Among all my snapshots, I''m interested about space consumption
of some
>> of them. Is it possible to calculate how much space would be freed if
>> I destroy only some of all snapshots?
>
> zfs list -t snapshot
>
> that''ll show you their sizes as well as other details
>
> Vennlige hilsener / Best regards
>
> roy
Okay, let''s think about this situation:

machine# zfs get used storage
NAME     PROPERTY  VALUE    SOURCE
storage  used      1,00G    -

It seems my storage consumes one gigabyte in total, and that sounds  
logical, since I''ve had one 512MB and two 256MB files in my pool, in  
different snapshots. There has been file removings, etc., but all of  
those three files are still there, stored in some snapshots. Now lets  
list snapshots:

machine# zfs list -t snapshot
NAME                 USED  AVAIL  REFER  MOUNTPOINT
storage at snapshot_1    18K      -   512M  -
storage at snapshot_2      0      -   769M  -
storage at snapshot_3      0      -   769M  -
storage at snapshot_4      0      -   769M  -
storage at snapshot_5      0      -   769M  -

This is the important moment. I would like to know how much space  
would become free if I destroy snapshots 4 and 5. Normally I cannot  
destroy them, but here, in this test I can. So lets find out:

machine# zfs destroy storage at snapshot_5
machine# zfs destroy storage at snapshot_4

And then check used space again

machine# zfs get used storage
NAME     PROPERTY  VALUE    SOURCE
storage  used      769M     -

About 256MB was released, but that number cannot be calculated from  
those that were given from "zfs list -t snapshot" (0, 0, 769M and  
769M). I also tried "zfs list -o all -t snapshot" but that did not  
give any interesting numbers.

I think this is because snapshots 4 and 5 are the only snapshots that  
contain one 256MB file. If they are examined one snapshot at a time,  
then it looks like the data is not unique, but if there would be  
possible to examine them "as a whole", then it would reveal that these
two snapshots are the only ones where the file exist. This would mean  
that when destroying both of them, the file would be destroyed and  
256MB would be freed.

Henrik Heino