There are a number of variables or options in Data ONTAP that are important to understand before configuring for thin provisioning.
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.
Guarantees
With flexible volumes, introduced with Data ONTAP 7.0, there is the concept of space guarantees, which allow the user to determine when space is reserved or allocated from the containing aggregate.
Volume—A guarantee of “volume” ensures that the amount of space required by the FlexVol volume is always available from its aggregate. This is the default setting for FlexVol volumes. With the space guarantee set to “volume” the space is subtracted, or reserved, from the aggregate’s available space at volume creation time. The space is reserved from the aggregate regardless of whether it is actually used for data storage or not.
The example shown here shows the creation of a 20GB volume. The df commands showing the space usage of the aggregate before and after the volume create command display how the 20GB is removed from the aggregate as soon as the volume is created, even though no data has actually been written to the volume.
netapp1> df -A -g aggr0
Aggregate total used avail capacity
aggr0 85GB 0GB 85GB 0%
aggr0/.snapshot 4GB 0GB 4GB 0%
netapp1> vol create flex0 aggr0 20g
Creation of volume 'flex0' with size 20g on hosting aggregate 'aggr0' has
completed.
netapp1> df -g /vol/flex0
Filesystem total used avail capacity Mounted on
/vol/flex0/ 16GB 0GB 16GB 0% /vol/flex0/
/vol/flex0/.snapshot 4GB 0GB 4GB 0% /vol/flex0/.snapshot
netapp1> df -A -g aggr0
Aggregate total used avail capacity
aggr0 85GB 20GB 65GB 23%
aggr0/.snapshot 4GB 0GB 4GB 0%
Since the space has already been reserved from the aggregate, write operations to the volume will not cause more space from the aggregate to be used.
None—A FlexVol volume with a guarantee of “none” reserves no space from the aggregate during volume creation. Space is first taken from the aggregate when data is actually written to the volume. The example here shows how, in contrast to the example above with the volume guarantee, the volume creation does not reduce used space in the aggregate. Even LUN creation, which by default has space reservation enabled, does not reserve space out of the aggregate. Write operations to space-reserved LUNs in a volume with guarantee=none will fail if the containing aggregate does not have enough available space. LUN reservation assure that the LUN has space in the volume but guarantee=none doesn’t assure that the volume has space in the aggregate.
netapp1> df -A -g aggr0
Aggregate total used avail capacity
aggr0 85GB 0GB 85GB 0%
aggr0/.snapshot 4GB 0GB 4GB 0%
netapp1>
netapp1> vol create noneflex -s none aggr0 20g
Creation of volume 'noneflex' with size 20g on hosting aggregate
'aggr0' has completed.
netapp1>
netapp1> df -g /vol/noneflex
Filesystem total used avail capacity Mounted on
/vol/noneflex/ 16GB 0GB 16GB 0% /vol/noneflex/
/vol/noneflex/.snapshot 4GB 0GB 4GB 0% /vol/noneflex/.snapshot
netapp1>
netapp1> df -A -g aggr0
Aggregate total used avail capacity
aggr0 85GB 0GB 85GB 0%
aggr0/.snapshot 4GB 0GB 4GB 0%
netapp1> lun create -s 10g -t windows /vol/noneflex/foo
Mon Nov 24 15:17:28 EST [array1: lun.vdisk.spaceReservationNotHonored:notice]:
Space reservations in noneflex are not being honored, either because the volume
space guarantee is set to 'none' or the guarantee is currently disabled due to
lack of space in the aggregate.
lun create: created a LUN of size: 10.0g (10742215680)
netapp1>
netapp1> df -g /vol/noneflex
Filesystem total used avail capacity Mounted on
/vol/noneflex/ 16GB 10GB 6GB 0% /vol/noneflex/
/vol/noneflex/.snapshot 4GB 0GB 4GB 0% /vol/noneflex/.snapshot
netapp1>
netapp1> df -A -g aggr0
Aggregate total used avail capacity
aggr0 85GB 0GB 85GB 0%
aggr0/.snapshot 4GB 0GB 4GB 0%
File—With guarantee=file the aggregate assures that space is always available for overwrites to space-reserved LUNs. Fractional reserve, a volume level option discussed later in this paper, is set to 100% and is not adjustable with this type of guarantee. The “file” guarantee is basically the same as the “none” guarantee with the exception that space reservations for LUNs and spacereserved files are honored. The example below looks the same as the previous example under with guarantee=none except in this example the LUN creation takes space from the aggregate because it is a space-reserved object. Since the space reservation is honored, the “lun create” command also doesn’t issue the warning shown in the previous example.
netapp1> df -A -g aggr0
Aggregate total used avail capacity
aggr0 85GB 0GB 85GB 0%
aggr0/.snapshot 4GB 0GB 4GB 0%
netapp1>
netapp1> vol create noneflex -s file aggr0 20g
Creation of volume 'noneflex' with size 20g on hosting aggregate
'aggr0' has completed.
cnrl1>
netapp1> df -g /vol/noneflex
Filesystem total used avail capacity Mounted on
/vol/noneflex/ 16GB 0GB 16GB 0 % /vol/noneflex/
/vol/noneflex/.snapshot 4GB 0GB 4GB 0%
/vol/noneflex/.snapshot
netapp1>
netapp1> df -A -g aggr0
Aggregate total used avail capacity
aggr0 85GB 0GB 85GB 0%
aggr0/.snapshot 4GB 0GB 4GB 0%
netapp1>
netapp1> lun create -s 10g -t windows /vol/noneflex/foo
lun create: created a LUN of size: 10.0g (10742215680)
netapp1>
netapp1> df -g /vol/noneflex
Filesystem total used avail capacity Mounted on
/vol/noneflex/ 16GB 10GB 6GB 0% /vol/noneflex/
/vol/noneflex/.snapshot 4GB 0GB 4GB 0% /vol/noneflex/.snapshot
netapp1>
netapp1> df -A -g aggr0
Aggregate total used avail capacity
aggr0 85GB 10GB 75GB 12%
aggr0/.snapshot 4GB 0GB 4GB 0%
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.
Guarantees
With flexible volumes, introduced with Data ONTAP 7.0, there is the concept of space guarantees, which allow the user to determine when space is reserved or allocated from the containing aggregate.
Volume—A guarantee of “volume” ensures that the amount of space required by the FlexVol volume is always available from its aggregate. This is the default setting for FlexVol volumes. With the space guarantee set to “volume” the space is subtracted, or reserved, from the aggregate’s available space at volume creation time. The space is reserved from the aggregate regardless of whether it is actually used for data storage or not.
The example shown here shows the creation of a 20GB volume. The df commands showing the space usage of the aggregate before and after the volume create command display how the 20GB is removed from the aggregate as soon as the volume is created, even though no data has actually been written to the volume.
netapp1> df -A -g aggr0
Aggregate total used avail capacity
aggr0 85GB 0GB 85GB 0%
aggr0/.snapshot 4GB 0GB 4GB 0%
netapp1> vol create flex0 aggr0 20g
Creation of volume 'flex0' with size 20g on hosting aggregate 'aggr0' has
completed.
netapp1> df -g /vol/flex0
Filesystem total used avail capacity Mounted on
/vol/flex0/ 16GB 0GB 16GB 0% /vol/flex0/
/vol/flex0/.snapshot 4GB 0GB 4GB 0% /vol/flex0/.snapshot
netapp1> df -A -g aggr0
Aggregate total used avail capacity
aggr0 85GB 20GB 65GB 23%
aggr0/.snapshot 4GB 0GB 4GB 0%
Since the space has already been reserved from the aggregate, write operations to the volume will not cause more space from the aggregate to be used.
None—A FlexVol volume with a guarantee of “none” reserves no space from the aggregate during volume creation. Space is first taken from the aggregate when data is actually written to the volume. The example here shows how, in contrast to the example above with the volume guarantee, the volume creation does not reduce used space in the aggregate. Even LUN creation, which by default has space reservation enabled, does not reserve space out of the aggregate. Write operations to space-reserved LUNs in a volume with guarantee=none will fail if the containing aggregate does not have enough available space. LUN reservation assure that the LUN has space in the volume but guarantee=none doesn’t assure that the volume has space in the aggregate.
netapp1> df -A -g aggr0
Aggregate total used avail capacity
aggr0 85GB 0GB 85GB 0%
aggr0/.snapshot 4GB 0GB 4GB 0%
netapp1>
netapp1> vol create noneflex -s none aggr0 20g
Creation of volume 'noneflex' with size 20g on hosting aggregate
'aggr0' has completed.
netapp1>
netapp1> df -g /vol/noneflex
Filesystem total used avail capacity Mounted on
/vol/noneflex/ 16GB 0GB 16GB 0% /vol/noneflex/
/vol/noneflex/.snapshot 4GB 0GB 4GB 0% /vol/noneflex/.snapshot
netapp1>
netapp1> df -A -g aggr0
Aggregate total used avail capacity
aggr0 85GB 0GB 85GB 0%
aggr0/.snapshot 4GB 0GB 4GB 0%
netapp1> lun create -s 10g -t windows /vol/noneflex/foo
Mon Nov 24 15:17:28 EST [array1: lun.vdisk.spaceReservationNotHonored:notice]:
Space reservations in noneflex are not being honored, either because the volume
space guarantee is set to 'none' or the guarantee is currently disabled due to
lack of space in the aggregate.
lun create: created a LUN of size: 10.0g (10742215680)
netapp1>
netapp1> df -g /vol/noneflex
Filesystem total used avail capacity Mounted on
/vol/noneflex/ 16GB 10GB 6GB 0% /vol/noneflex/
/vol/noneflex/.snapshot 4GB 0GB 4GB 0% /vol/noneflex/.snapshot
netapp1>
netapp1> df -A -g aggr0
Aggregate total used avail capacity
aggr0 85GB 0GB 85GB 0%
aggr0/.snapshot 4GB 0GB 4GB 0%
File—With guarantee=file the aggregate assures that space is always available for overwrites to space-reserved LUNs. Fractional reserve, a volume level option discussed later in this paper, is set to 100% and is not adjustable with this type of guarantee. The “file” guarantee is basically the same as the “none” guarantee with the exception that space reservations for LUNs and spacereserved files are honored. The example below looks the same as the previous example under with guarantee=none except in this example the LUN creation takes space from the aggregate because it is a space-reserved object. Since the space reservation is honored, the “lun create” command also doesn’t issue the warning shown in the previous example.
netapp1> df -A -g aggr0
Aggregate total used avail capacity
aggr0 85GB 0GB 85GB 0%
aggr0/.snapshot 4GB 0GB 4GB 0%
netapp1>
netapp1> vol create noneflex -s file aggr0 20g
Creation of volume 'noneflex' with size 20g on hosting aggregate
'aggr0' has completed.
cnrl1>
netapp1> df -g /vol/noneflex
Filesystem total used avail capacity Mounted on
/vol/noneflex/ 16GB 0GB 16GB 0 % /vol/noneflex/
/vol/noneflex/.snapshot 4GB 0GB 4GB 0%
/vol/noneflex/.snapshot
netapp1>
netapp1> df -A -g aggr0
Aggregate total used avail capacity
aggr0 85GB 0GB 85GB 0%
aggr0/.snapshot 4GB 0GB 4GB 0%
netapp1>
netapp1> lun create -s 10g -t windows /vol/noneflex/foo
lun create: created a LUN of size: 10.0g (10742215680)
netapp1>
netapp1> df -g /vol/noneflex
Filesystem total used avail capacity Mounted on
/vol/noneflex/ 16GB 10GB 6GB 0% /vol/noneflex/
/vol/noneflex/.snapshot 4GB 0GB 4GB 0% /vol/noneflex/.snapshot
netapp1>
netapp1> df -A -g aggr0
Aggregate total used avail capacity
aggr0 85GB 10GB 75GB 12%
aggr0/.snapshot 4GB 0GB 4GB 0%
