Space management for volumes and files

What space management is

The space management capabilities of Data ONTAP allow you to configure your storage systems to provide the storage availability required by the users and applications accessing the system, while using your available storage as effectively as possible.

Data ONTAP provides space management using the following capabilities:

Space guarantees
This capability is available only for FlexVol volumes.

For more information, see Space guarantees.
Space reservations
For more information, see Space reservations and your Block Access Management Guide.
Fractional reserve
This capability is an extension of space reservations that is new for Data ONTAP 7.0.

For more information, see Fractional reserve and your Block Access Management Guide.
Space management policy
This capability allows you to automatically reclaim space for a FlexVol volume when that FlexVol volume is nearly full.

For more information, see Space management policies and your Block Access Management Guide.

Space management and files

Space reservations, fractional reserve, and the space management policy are designed primarily for use with LUNs. Therefore, they are explained in greater detail in your Block Access Management Guide. If you want to use these space management capabilities for files, consult those guides, keeping in mind that files are managed by Data ONTAP exactly the same as LUNs, except that space reservations are enabled for LUNs by default, whereas space reservations must be explicitly enabled for files.

What kind of space management to use

The following table can help you determine which space management capabilities best suit your requirements.

If...	Then use...	Typical usage	Notes
You want management simplicity You have been using a version of Data ONTAP earlier than 7.0 and want to continue to manage your space the same way	FlexVol volumes with space guarantee = `volume` Traditional volumes	NAS file systems	This is the easiest option to administer. As long as you have sufficient free space in the volume, writes to any file in this volume will always succeed. For more information about space guarantees, see Space guarantees.
Writes to certain files must always succeed You want to overcommit your aggregate	FlexVol volumes with space guarantee = `file` OR Traditional volume AND space reservation enabled for files that require writes to succeed	LUNs Databases	This option enables you to guarantee writes to specific files. For more information about space guarantees, see Space guarantees. For more information about space reservations, see Space reservations and your Block Access Management Guide.
You need even more effective storage usage than file space reservation provides You actively monitor available space on your volume and can take corrective action when needed Snapshots are short-lived Your rate of data overwrite is relatively predictable and low	FlexVol volumes with space guarantee = `volume` OR Traditional volume AND Space reservation on for files that require writes to succeed AND Fractional reserve < 100%	LUNs (with active space monitoring) Databases (with active space monitoring)	With fractional reserve <100%, it is possible to use up all available space, even with space reservations on. Before enabling this option, be sure either that you can accept failed writes or that you have correctly calculated and anticipated storage and snapshot usage. For more information, see Fractional reserve and your Block Access Management Guide.
You want to overcommit your aggregate You actively monitor available space on your aggregate and can take corrective action when needed	FlexVol volumes with space guarantee = `none`	Storage providers who need to provide storage that they know will not immediately be used Storage providers who need to allow available space to be dynamically shared between volumes	With an overcommitted aggregate, writes can fail due to insufficient space. For more information about aggregate overcommitment, see Aggregate overcommitment.