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Autodelete deltes too many snapshots

I have a volume with 180 GB and a LUN with 100 GB. The volume has this autodelete settings:

state                           : on

commitment                      : try

trigger                         : volume

target_free_space               : 20%

delete_order                    : oldest_first

defer_delete                    : user_created

prefix                          : (not specified)

destroy_list                    : none

But snapshots are deleted until the volume reaches 98 % capacity. Here is the snapshot history:

First day:

NameAccessTimeDTTotal
exchsnap__ecorreo__recent22/05/2011 20:02997,3
exchsnap__ecorreo_05-21-2011_22.01.24__daily21/05/2011 20:02875,7
exchsnap__ecorreo_05-20-2011_22.02.22__daily20/05/2011 20:031628,9
exchsnap__ecorreo_05-19-2011_22.02.14__daily19/05/2011 20:033076,2
exchsnap__ecorreo_05-18-2011_22.02.10__daily18/05/2011 20:022675,6
exchsnap__ecorreo_05-17-2011_22.01.21__daily17/05/2011 20:022882,9
exchsnap__ecorreo_05-16-2011_22.35.32__daily16/05/2011 20:362972,3
exchsnap__ecorreo_05-15-2011_22.02.05__daily15/05/2011 20:022054,8
exchsnap__ecorreo_05-14-2011_22.02.25__daily14/05/2011 20:031308,2
exchsnap__ecorreo_05-13-2011_22.01.20__daily13/05/2011 20:022243,1
exchsnap__ecorreo_05-12-2011_22.01.20__daily12/05/2011 20:021990,5
exchsnap__ecorreo_05-11-2011_22.01.19__daily11/05/2011 20:022144,1
exchsnap__ecorreo_05-10-2011_22.01.16__daily10/05/2011 20:013123,4
exchsnap__ecorreo_05-09-2011_22.01.17__daily09/05/2011 20:012522,1
exchsnap__ecorreo_05-08-2011_22.01.20__daily08/05/2011 20:022668,2
exchsnap__ecorreo_05-07-2011_22.01.22__daily07/05/2011 20:021598,2
exchsnap__ecorreo_05-06-2011_22.01.19__daily06/05/2011 20:022251,7
exchsnap__ecorreo_05-05-2011_22.01.21__daily05/05/2011 20:023889
exchsnap__ecorreo_05-04-2011_22.01.20__daily04/05/2011 20:022987,9
exchsnap__ecorreo_05-03-2011_22.01.20__daily03/05/2011 20:022744,6
exchsnap__ecorreo_05-02-2011_22.01.18__daily02/05/2011 20:012513,7
exchsnap__ecorreo_05-01-2011_22.01.28__daily01/05/2011 20:022164,9
exchsnap__ecorreo_04-30-2011_22.01.21__daily30/04/2011 20:02590,7
exchsnap__ecorreo_04-29-2011_22.01.27__daily29/04/2011 20:021481,3
exchsnap__ecorreo_04-28-2011_22.01.19__daily28/04/2011 20:023928,4
exchsnap__ecorreo_04-27-2011_22.01.55__daily27/04/2011 20:024506,8
exchsnap__ecorreo_04-26-2011_22.01.17__daily26/04/2011 20:012181,3
exchsnap__ecorreo_04-25-2011_22.01.27__daily25/04/2011 20:022326,4
15.0 GB free on volume

Next day:

NameAccessTimeDTTotal
exchsnap__ecorreo__recent23/05/2011 20:021264,1
exchsnap__ecorreo_05-22-2011_22.01.19__daily22/05/2011 20:022713,7
exchsnap__ecorreo_05-21-2011_22.01.24__daily21/05/2011 20:02875,7
exchsnap__ecorreo_05-20-2011_22.02.22__daily20/05/2011 20:031628,9
exchsnap__ecorreo_05-19-2011_22.02.14__daily19/05/2011 20:033076,2
exchsnap__ecorreo_05-18-2011_22.02.10__daily18/05/2011 20:022675,6
exchsnap__ecorreo_05-17-2011_22.01.21__daily17/05/2011 20:022882,9
exchsnap__ecorreo_05-16-2011_22.35.32__daily16/05/2011 20:362972,3
exchsnap__ecorreo_05-15-2011_22.02.05__daily15/05/2011 20:022054,8
exchsnap__ecorreo_05-14-2011_22.02.25__daily14/05/2011 20:031308,2
exchsnap__ecorreo_05-13-2011_22.01.20__daily13/05/2011 20:022243,1
exchsnap__ecorreo_05-12-2011_22.01.20__daily12/05/2011 20:021990,5
exchsnap__ecorreo_05-11-2011_22.01.19__daily11/05/2011 20:022144,1
exchsnap__ecorreo_05-10-2011_22.01.16__daily10/05/2011 20:013123,4
exchsnap__ecorreo_05-09-2011_22.01.17__daily09/05/2011 20:012522,1
exchsnap__ecorreo_05-08-2011_22.01.20__daily08/05/2011 20:022668,2
exchsnap__ecorreo_05-07-2011_22.01.22__daily07/05/2011 20:021598,2
exchsnap__ecorreo_05-06-2011_22.01.19__daily06/05/2011 20:022251,7
exchsnap__ecorreo_05-05-2011_22.01.21__daily05/05/2011 20:023889
exchsnap__ecorreo_05-04-2011_22.01.20__daily04/05/2011 20:022987,9
34.0 GB free on volume


With an increment of 2.9 GB, snapshots are deleted about 15 GB. The volume had 15 GB of free space.

Is working wrong DATA ONTAP??

Autodelete deltes too many snapshots

This is the correct behaviour.

You say you want Autodelete to free up 20% in your volume. Your volume is 180gb. 20% of 180gb is ~36gb. So snap autodelete deletes until 36gb are free. If you have "only" 34gb free, it means that autodelete actually stopped too early, it should have deleted one more snapshot to reach 36gb free space ;-)

If you want to delete fewer snapshots, simply set the target_free_space option lower. You shouldn't set it below 10gb though, as that is the minimum "target_free_space" value for a volume of that size. So if you set target_free_space to 10% (the minimum), you will make autodelete free up only ~18gb on the volume. You cannot, however, control the point where Autodelete starts. For a volume of 180g it always starts when the volume is 92% full and deletes untill target_free_space percent are free.

Hope that helps

-Michael

Autodelete deltes too many snapshots

Thanks Michael,

     I think that I had made wrong my calculations. This behavior is all right.