Fractional reserve & space reserve

Fractional reserve:

Fractional reserve is the volume options which reserve the space inside the volume for the snapshot overwrites. By default it is 100%, but by autodelete functionality NetApp recommend to keep the fractional reserve to =0, as soon as the first snapshot copy is created the NetApp automatically creates the fractional reserve and start using it only when whole volume space gets filled. This reserved space is used only when the volume is 100% full.

The amount of reserve space can be seen by –r options in df cmd.

Snap reserve:

Snap reserve is the space reserved for storing the snap shot copies as the snapshot copies also required space to get stored so they use the snap reserve space, by default it is20% of the volume space. As the snap reserve space gets filled it automatically start using the space from the volume. So we can say snap reserve is actually the logical separation space from the volume.

Space reclamation:

Space reclamation is the process of reclaiming back the space of the lun on storage side. For exp suppose we have filled the 100 gb lun with 50 gb of the data so the storage usage of lun in host side can be seen as 50% utilized and in storage side also it will show 50% utilized , but when we delete the data from the lun for exp we deleted all the 50 gb of data from the lun , the storage utilization on host side will be shown as 0% utilized but from storage end it will show 50% utilized , so we use the space reclamation to reclaim back the free space on storage side. Snapdrive work good on reclaiming back the data from storage end.

Lun reservation:

LUN reservation (not to be confused with SCSI2 or 3 logical unit locking reservations) determines when space for the LUN is reserved or allocated from the volume. With reservations enabled (default) the space is subtracted from the volume total when the LUN is created. For example, if a 20GB LUN is created in a volume having 80GB of free space, the free space will go to 60GB free space at the time the LUN is created even though no writes have been performed to the LUN. If reservations are disabled, space is first taken out of the volume as writes to the LUN are performed. If the 20GB LUN was created without LUN space reservation enabled, the free space in the volume would remain at 80GB and would only go down as data was written to the LUN.