Space guarantees

What space guarantees are

Space guarantees on a FlexVol volume ensure that writes to a specified FlexVol volume or writes to files with space reservations enabled do not fail because of lack of available space in the containing aggregate.

Other operations such as creation of snapshots or new volumes in the containing aggregate can occur only if there is enough available uncommitted space in that aggregate; other operations are restricted from using space already committed to another volume.

When the uncommitted space in an aggregate is exhausted, only writes to volumes or files in that aggregate with space guarantees are guaranteed to succeed.

A space guarantee of volume preallocates space in the aggregate for the volume. The preallocated space cannot be allocated to any other volume in that aggregate.
The space management for a FlexVol volume with space guarantee of volume is equivalent to a traditional volume, or all volumes in versions of Data ONTAP earlier than 7.0.
A space guarantee of file preallocates space in the volume so that any file in the volume with space reservation enabled can be completely rewritten, even if its blocks are pinned for a snapshot.
For more information on file space reservation see Space reservations.
A FlexVol volume with a space guarantee of none reserves no extra space; writes to LUNs or files contained by that volume could fail if the containing aggregate does not have enough available space to accommodate the write.
Note
Because out-of-space errors are unexpected in a CIFS environment, do not set space guarantee to none for volumes accessed using CIFS.

Space guarantee is an attribute of the volume. It is persistent across storage system reboots, takeovers, and givebacks, but it does not persist through reversions to versions of Data ONTAP earlier than 7.0.

Space guarantees and volume status

Space guarantees are honored only for online volumes. If you take a volume offline, any committed but unused space for that volume becomes available for other volumes in that aggregate. When you bring that volume back online, if there is not sufficient available space in the aggregate to fulfill its space guarantees, you must use the force (-f) option, and the volume's space guarantees are disabled.

For more information, see Bringing a volume online in an overcommitted aggregate.

Traditional volumes and space management

Traditional volumes provide the same space guarantee as FlexVol volumes with space guarantee of volume. To guarantee that writes to a specific file in a traditional volume will always succeed, you need to enable space reservations for that file. (LUNs have space reservations enabled by default.)

For more information about space reservations, see Space reservations.

Specifying space guarantee at FlexVol volume creation time

To specify the space guarantee for a volume at creation time, complete the following steps.

Note
To create a FlexVol volume with space guarantee of volume, you can ignore the guarantee parameter, because volume is the default.

Step	Action
1	Enter the following command: *vol create f_vol_name* aggr_name -s {volume\|file\|none} size{k\|m\|g\|t}** f_vol_name is the name for the new FlexVol volume (without the /vol/ prefix). This name must be different from all other volume names on the storage system. aggr_name is the containing aggregate for this FlexVol volume. `-s` specifies the space guarantee to be used for this volume. The possible values are `{volume\|file\|none}`. The default value is `volume`. size `{k\|m\|g\|t}` specifies the maximum volume size in kilobytes, megabytes, gigabytes, or terabytes. For example, you would enter `4m` to indicate four megabytes. If you do not specify a unit, size is considered to be in bytes and rounded up to the nearest multiple of 4 KB.
2	To confirm that the space guarantee is set, enter the following command: vol options f_vol_name

Step

Action

Enter the following command:

vol create f_vol_name aggr_name -s {volume|file|none} 
size{k|m|g|t}

f_vol_name is the name for the new FlexVol volume (without the /vol/ prefix). This name must be different from all other volume names on the storage system.

aggr_name is the containing aggregate for this FlexVol volume.

-s specifies the space guarantee to be used for this volume. The possible values are {volume|file|none}. The default value is volume.

size {k|m|g|t} specifies the maximum volume size in kilobytes, megabytes, gigabytes, or terabytes. For example, you would enter 4m to indicate four megabytes. If you do not specify a unit, size is considered to be in bytes and rounded up to the nearest multiple of 4 KB.

To confirm that the space guarantee is set, enter the following command:

vol options f_vol_name

Changing space guarantee for existing volumes

To change the space guarantee for an existing FlexVol volume, complete the following steps.

Step	Action
1	Enter the following command: vol options f_vol_name guarantee guarantee_value f_vol_name is the name of the FlexVol volume whose space guarantee you want to change. guarantee_value is the space guarantee you want to assign to this volume. The possible values are `volume`, `file`, and `none`. Note If there is insufficient space in the aggregate to honor the space guarantee you want to change to, the command succeeds, but a warning message is printed and the space guarantee for that volume is disabled.
2	To confirm that the space guarantee is set, enter the following command: vol options f_vol_name

Step

Action

Enter the following command:

vol options f_vol_name guarantee guarantee_value

f_vol_name is the name of the FlexVol volume whose space guarantee you want to change.

guarantee_value is the space guarantee you want to assign to this volume. The possible values are volume, file, and none.

Note
If there is insufficient space in the aggregate to honor the space guarantee you want to change to, the command succeeds, but a warning message is printed and the space guarantee for that volume is disabled.

To confirm that the space guarantee is set, enter the following command:

vol options f_vol_name

Aggregate overcommitment

Aggregate overcommitment provides flexibility to the storage provider. Using aggregate overcommitment, you can appear to provide more storage than is actually available from a given aggregate. This could be useful if you are asked to provide greater amounts of storage than you know will be used immediately. Alternatively, if you have several volumes that sometimes need to grow temporarily, the volumes can dynamically share the available space with each other.

To use aggregate overcommitment, you create FlexVol volumes with a space guarantee of none or file. With a space guarantee of none or file, the volume size is not limited by the aggregate size. In fact, each volume could, if required, be larger than the containing aggregate. The storage provided by the aggregate is used up only as LUNs are created or data is appended to files in the volumes.

Of course, when the aggregate is overcommitted, it is possible for these types of writes to fail due to lack of available space:

Writes to any volume with space guarantee of none
Writes to any file that does not have space reservations enabled and that is in a volume with space guarantee of file

Therefore, if you have overcommitted your aggregate, you must monitor your available space and add storage to the aggregate as needed to avoid write errors due to insufficient space.

Note
Because out-of-space errors are unexpected in a CIFS environment, do not set space guarantee to none for volumes accessed using CIFS.

Bringing a volume online in an overcommitted aggregate

When you take a FlexVol volume offline, it releases its allocation of storage space in its containing aggregate. Storage allocation for other volumes in that aggregate while that volume is offline can result in that storage being used. When you bring the volume back online, if there is insufficient space in the aggregate to fulfill the space guarantee of that volume, the normal online command fails unless you force the volume online by using the -f flag.

Attention
When you force a FlexVol volume to come online due to insufficient space, the space guarantees for that volume are disabled. That means that attempts to write to that volume could fail due to insufficient available space. In environments that are sensitive to that error, such as CIFS or LUNs, forcing a volume online should be avoided if possible.

When you make sufficient space available to the aggregate, the space guarantees for the volume are automatically re-enabled.

Note
FlexCache volumes cannot be brought online if there is insufficient space in the aggregate to fulfill their space guarantee.

For more information about FlexCache volumes, see Managing FlexCache volumes.

To bring a FlexVol volume online when there is insufficient storage space to fulfill its space guarantees, complete the following step.

Step	Action
1	Enter the following command: `vol online` vol_name `-f` vol_name is the name of the volume you want to force online.