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NetApp Data Planning

With the introduction of all kind of options in their filers NetApp provides several ways to plan for your data consumption. In this article I'll discuss a few of the most confusing options and clarify them and their interaction with each other by showing an example.

Fractional Reserve

Note: Since Operation Manager 3.7 this option might be referred to as 'Overwrite Reserved Space'.

* Default = 100%

  • Can not be set using the http(s) console
  • Can be set using System Manager: Filer → Storage → Volumes → Edit → Space Settings
  • Can also be set using the console:
First check the current setting:
prd-filer2*> vol options VOL_NAME
nosnap=off, nosnapdir=off, minra=on, no_atime_update=on, nvfail=off,
ignore_inconsistent=off, snapmirrored=off, create_ucode=on,
convert_ucode=off, maxdirsize=41861, schedsnapname=ordinal,
fs_size_fixed=off, compression=off, guarantee=volume, svo_enable=off,
svo_checksum=off, svo_allow_rman=off, svo_reject_errors=off,
no_i2p=off, fractional_reserve=100, extent=off, try_first=volume_grow,
read_realloc=off, snapshot_clone_dependency=off

Than set the option like this:
prd-filer2*> vol options VOL_NAME fractional_reserve 20

Fractional reserve is a volume option that determines how much space Data ONTAP will reserve for Snapshot overwrite data for LUNs and space-reserved files to be used after all other space in the volume is used.

Snap Reserve

  • Default = 20 %
  • Can be set using the http(s) console: Volumes → Snapshots → Configure → Snapshot Reserve
  • Can be set using System Manager: Filer → Storage → Volumes → Snapshot → Configure

This variable is set at the volume level and is set as a percentage of the volume. Data ONTAP removes the defined percentage of volume from being available for configuring LUNs or for file usage with CIFS or NFS. As Snapshot copies need space, they consume space in the snap reserve area. By default after the snap reserve area is filled, the Snapshot copies start to take space from the general volume. Of course, because of WAFL write anywhere technology, snap reserve doesn’t actually reserve specific physical blocks for Snapshot usage and can be thought of as a logical space accounting mechanism.

Interaction Between SnapShots, Fractional Reserve, Snap Reserve, Volumes and LUNs

To understand the interaction between snapshots, reserved space, and volumes/LUN read the example below. In this example 'lun reserve', 'fractional_reserve', and 'snap reserve' all have been left at their default values.


  • Step #1: A 100GB volume is created. The available space is shown as 80GB since by default there is a 20% snap reserve area.
  • Step #2: A 30GB LUN called 'TestLUN' is created. Because LUNs are by default space-reserved objects, the 30GB is immediately taken from the available space from the volume.
  • Step #3: The LUN is filled with data and since the space was already reserved there is no change in reported space usage.


  • Step #4: A Snapshot is created causing Data ONTAP to reserve 30GB of space from the volume, ensuring that there is space for Snapshot data even if the LUN is completely overwritten. Note that if fractional_reserve was set to less than 100, the amount of space reserved would be less than 30GB.
  • Step #5: 10GB of data is overwritten in the TestLUN. Since a Snapshot exists, overwriting data in a LUN means that space will start to be consumed for Snapshot data. The first space used for this is from the Snapshot reserve area. This is seen in the df output in the /vol/myvol/.Snapshot line where 10GB is now shown as used.
  • Step #6: Another 10GB of data is overwritten in the TestLUN. This uses up the rest of the Snapshot reserve area, which now shows up as full.


  • Step #7: Another 10GB of data is overwritten in the TestLUN. Even though the Snapshot reserve area is already full, the writes are allowed to happen and no Snapshot copies are lost or corrupted because there is still available space in the volume. These writes simply take space from the available space in the volume, which goes down from 20GB as shown in the last step to 10GB after Step #7. The Snapshot space usage that goes above the amount of snap reserve can be considered counted twice in the df command output since it shows up in the used column of both the general volume and the snap reserve space. It’s interesting to note that the used column df output for the snap reserve area can go above 100%.
  • Step #8: Another 10GB of data is written to the TestLUN. In this case, because in the previous steps the entire LUN was already overwritten, no additional Snapshot data requires space. There is no change to the space usage.
  • Step #9: A second Snapshot is created. Unlike the creation of the first Snapshot, the creation of more Snapshot copies doesn’t cause any immediate changes to the space usage. The creation of additional Snapshot copies will not directly cause space to be consumed. However, as the count of Snapshot copies increases, the possibility that the Snapshot reserve space will be needed goes up, since multiple overwrites of a block are more likely to be caught in separate Snapshot copies.


  • Step #10: 10GB of the LUN is overwritten. Since a Snapshot copy was just created in the previous step, it doesn’t matter which 10GB of the LUN is overwritten, since space is still required to hold the Snapshot data. In this case, the 10GB of available space is taken out of the volume. At this point the volume reports as full and it is not possible to create any more LUNs, create any files via NFS/CIFS, create more Snapshot copies, or write to non-space-reserved files.
  • Step #11: An attempt is made to create a third Snapshot. This operation fails because the volume is full.
  • Step #12: Another 10GB of data is overwritten in the TestLUN. Even though the volume is full, the writes to the TestLUN are successful and the Snapshot copies are not lost because there is still space available in the reserve area. The only change to the df command output will be in the used column of the snap reserve area. Because the fractional_reserve is set to 100%, writes to the LUN will never fail for lack of space. If the fractional_reserve were set to less than 100% and overwrites continued, at some point writes to the LUN might not be possible because of lack of space.


Note: This option is also known as Autogrow

* Default growth increment = 5% of volume size at creation

  • Default maximum size to grow to = 120% of volume size at creation

This volume setting (available in Data ONTAP 7.1 later) defines whether a volume should automatically grow to avoid filling up to capacity. This option is available only for flexible volumes. It is possible to define how fast the volume should grow with the '-i' option. The default growth increment is 5% of the volume size at creation. It is also possible to define how large the volume is allowed to grow to with the '-m' option. If volume autosize is enabled, the default maximum size to grow to is 120% of the original volume size.

Viewing volume autosize option:
acc-filer1> vol autosize vol0
Volume autosize is currently ON for volume 'vol0'.
The volume is set to grow to a maximum of 80 GB, in increments of 2 GB.

Setting volume autosize option:
acc-filer1> vol autosize vol0 -m 80g -i 2g on
vol autosize: Flexible volume 'vol0' autosize settings UPDATED.

Viewing results when resizing volumes when autosize is on:
acc-filer1> vol size vol0 80g
vol size: Flexible volume 'vol0' size set to 80g.
vol size: Warning: Flexible volume 'vol0' autosize policy with limit (83886080KB) is overridden by this new volume size setting. Recommend disabling the autosize policy.
acc-filer1> vol size vol0 50g
vol size: Flexible volume 'vol0' size set to 50g.
vol size: Warning: Flexible volume 'vol0' autosize limit (83886080KB) is more than the new size of the volume.
acc-filer1> vol autosize vol0
Volume autosize is currently ON for volume 'vol0'.
The volume is set to grow to a maximum of 80 GB, in increments of 2 GB.


  • Default target_free_space = 80%

This volume setting (available in Data ONTAP 7.1 and later) allows Data ONTAP to delete Snapshot copies if a threshold is met. This threshold is called a 'trigger' and can be set so that Snapshot copies will be automatically deleted under one of the following conditions:

  • Volume: The volume is near full. This is reported in the first line reported for each volume in the 'df' command. It should be noted that the volume can be full even though there might still be space in the snap_reserve areas.
  • Snap_reserve: The snap reserve space is near full.
  • Space_reserve: The overwrite reserved space is full. This is the space determined by the LUNs with space reservations enabled and the fractional_reserve option. The reserve space will never be filled until both the volume and the snap_reserve areas are full.

It is strongly recommended to set the trigger to volume.
The order in which Snapshot copies are deleted is determined by the following three options:

  • Delete_order: This option determines whether the oldest or newest Snapshot copies should be deleted first.
  • Defer_deleted: This option allows the user to define a group of Snapshot copies that should first be deleted when no other Snapshot copies are available. It is possible to defer the deletion of user created Snapshot copies, scheduled Snapshot copies or Snapshot copies beginning with a configurable prefix.
  • Commitment: This option determines how Snapshot copies used for SnapMirror® and dump operations should be handled.
    • If set to 'try', it will only delete these Snapshot copies if they are not locked.
    • If set to 'disrupt', these Snapshot copies will be deleted even if they are locked. These will always be the last Snapshot copies deleted. In many configurations, deleting the last SnapMirror Snapshot copy is not desired because a new full baseline copy will be required to resume mirroring operations. If, for example, the source and destination are at different sites, re-creating this baseline can be a time-consuming and costly process.

The algorithm to determine which Snapshot copy to delete will first look for a Snapshot that does not lie in the 'defer_delete' criteria and use the 'delete_order' to determine whether to delete the oldest or more recent Snapshot. If no such Snapshot copy is found, the 'defer_delete' criteria will be ignored in the selection process. If a Snapshot copy is still not available for deletion, then the SnapMirror and dump Snapshot copies will be targeted depending on the 'commit' option. Snapshot copies will stop being deleted when the free space in the trigger criteria reaches the value of the target_free_space variable, which defaults to 80%.
If both autosize and autodelete are enabled and the autodelete trigger is set to 'volume', the 'try_first' volume option will determine whether a volume grow or Snapshot copy delete will be attempted first.

Best Practice With Snapshot enabled

Autogrow Configuration

Option Setting
guarantee volume
LUN reservation enabled
fractional_reserve 0%
snap_reserve 0%
autodelete volume / oldest_first
autogrow on
try_first volume_grow

The big advantage of this configuration is that it takes advantage of using the free space in the aggregate as a shared pool of available space. Since a guarantee of volume is being used, a different level of thin provisioning per application can easily be achieved through individually sizing the volumes and tuning how much each volume is allowed to grow. Space usage is also very easy to monitor and understand by simply looking at the volume and aggregate usage.
As with all configurations using a shared free space, the volumes are not 100% independent of one another since they are competing for that space when they need to grow. Meaning that if other volumes have already consumed the space in the aggregate it might not be possible for a volume to grow.
Care needs to be taken if clones (FlexClone or LUN clones) are being created as Snapshot copies which are backing this clones cannot currently be deleted by the autodelete functionality. This risk can be avoided if the volume is sized at 2X or greater of the LUN size and there is only one locked snapshot.

Autodelete Configuration

Option Setting
guarantee volume
LUN reservation enabled
fractional_reserve 0%
snap_reserve 0%
autodelete volume / oldest_first
autogrow off
try_first snap_delete

This configuration is the same as the autogrow configuration with the removal of the autogrow functionality. This configuration makes it a priority to keep the LUNs accessible over maintaining Snapshot copies. If the volume starts to fill up Snapshot copies will be deleted to allow the LUNs to remain online. One of the advantages of this configuration is that it is easy to monitor and understand the space usage by just monitoring the volume space usage. The volumes are also independent of each other, meaning each application can be tuned independently. This configuration also has the advantage that only the oldest Snapshot copies are deleted while the most recent ones are maintained. The disadvantage of this configuration is that it doesn’t use the available space in the aggregate as a shared pool of available space.
As with the previous configuration, care needs to be taken if clones (FlexClone or LUN clones) are being created as Snapshot copies which are backing these clones cannot currently be deleted by the autodelete functionality.

Setting Options

These are the commands on the commandline of the filer to set the options from the best practices guideline we use:

vol options <volname> fractional_reserve 0
snap autodelete <volname> trigger volume
snap autodelete <volname> delete_order oldest_first
snap autodelete <volname> defer_delete none
snap autodelete <volname> target_free_space 10
snap autodelete <volname> on
vol options <volname> try_first volume_grow
vol autosize <volname> -m 80g -i 2g on
Note: Please note that the volume name is case sensitive.

Note: These are the available options for snap autodelete:

snap autodelete <vol-name> <option> <value>...
Supported options and corresponding values:
        commitment              try | disrupt | destroy
        trigger                 volume | snap_reserve | space_reserve
        target_free_space       1-100
        delete_order            oldest_first | newest_first
        defer_delete            scheduled | user_created | prefix | none
        prefix                  <string>
        destroy_list            lun_clone | vol_clone | cifs_share | none

Snapshots Configuration

The best practice outlined above is for volumes with snapshots enabled. In our environment that counts for all volumes containing boot luns, and for the vol0 volumes. Also volumes with snapmirror enabled will have snapshots although those snapshots will be automatically maintained and do not need further configuration.

This is the snapshot configuration as we use it:

  • Snapshot reserve = 0%
  • Snapshot directory visible = on
  • Snapshots scheduled = on
  • Snapshots to keep: 1 weekly, 2 nightly, 2 hourly
  • Snapshot times: every 6 hours

The weekly snapshot might grow considerately over time so monitor the amount of space occupied by the snapshots.


Above article is a simplification especially for my knowledge and environment from this pdf.

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netappdataplanning.txt · Last modified: 2021/09/23 22:24 (external edit)