Default is space reservation enabled. Also when you're not changing anything and you're going to use snapshots then you minimum need twice the LUN space inside the volume (because fractional reserve is 100%).
When you disable space reservation, you've to monitor your available space. For example if you have a empty LUN (with space reservation disabled) of 60 GB and placed this inside a volume of 20 GB, then you can write 20 GB inside the LUN without a problem. However, when you write a single byte above this 20 GB then LUN goes offline.
A solution for this is enabling auto grow on the volume. The only thing is that you have to monitor your aggregate all the time.
When you have a LUN withspace reserve enabled then after the creation of the LUN your volume is also 50GB filled. When you're not using snapshots the volume stays filled for 50 GB.When you enable snapshots then you standard need double the 50 GB (100 GB).
The reason of this is thatstandard the volume option fractional reserve is 100%. This option ensures(protects) that you can change 100% of the data inside the LUN (compared withthe snapshot). For example the following scenario:
1) you create a volume of100 GB and a LUN of 50 GB
2) execute at the CLI thecommand df and check that the volume is filled for 50%
3) put 50 GB of data insidethe LUN
4) create a snapshot
5) execute at the CLI thecommand df and check that the volume is filled for 100%
6) delete all the data onthe LUN and copy new data on it
You can play with thefractional reserve with or without adjusting lun space reservation. These 2options will give you several levels of thin provisioning. At multiplecustomers of us we played with these options. But mostly we're disablingfractional reserve and enabling lun space reservation. The real thinprovisioning is setting up fractional reserve at 0% and lun space reservationdisabled. With both methods it is advisable to enable volume growths and/orauto deletion of snapshots.
When not enabling auto growor snapshot deletion there is a big change that LUN's can go offline becausethe changes at the LUN cannot be written to the file system.
im trying o understand but not getting it. by default i think the snapshot reservation is 20% and space reservation is checked. if i set snapshot resevation to 0 and uncheck space reservation. then a 50gb lun us a 50gb lun. then what i see within esx will refect what i see in the filer right?
where does fractional reserve come into play? or where is this option?
By default it is a best practice to disable the complete snapshot reservation and schedule. If you disable snapshot resevation and uncheck space reservation then VMware still sees the 50 GB LUN, however the space on the NetApp is empty (when you just create the LUn and VMFS file system).
Fractional Reserve come into play when you're going to use snapshots on a volume with a LUN. The fractional reserve option is a volume option.
When you take a look at the following manual fractional reserve is explained: Block Access Management Guide for iSCSI and FC (page 33 and further)