A Temporary Writeable Snapshot would be analogous to a read/write snapshot (a mountable snap that allows for writes, but the writes are not preserved when the snapshot is destroyed).  Temporary Writeable Snapshot normally requires the same amount of space as the source LUN in order to function since it is "Fully Provisioned" by default.  You can make TWS's "Virtually Provisioned" by enabling the setting "nbs.sparseTws" on the datamover.  With that flag enabled, Temporary Writeable Snapshots should act like normal snapshots in that the only disk space requirement is for writes occurring to the LUN and/or Snapshot while the snapshot is active.  However, for a fully provisioned source LUN, the first snapshot requires 100% space to be reserved (virtually provisioned LUNs don't require this)  Examples below with both settings. 100GB LUN, 80GB of data in LUN, 20GB change per day, 2 snapshots per day, 1 writeable snapshot per day, 5GB data written to snapshot per day  nbs.sparseTWS = 1 (Virtually provisioned TWS, Virtually Provisioned Source LUN) Filesystem size = 80GB Virtually Provisioned iSCSI LUN, + 20GB of change on source LUN, + 5GB for TWS snapshot = 105GB total  nbs.sparseTWS = 1 (Virtually provisioned TWS, Fully Provisioned Source LUN) Filesystem size = 100GB Fully provisioned iSCSI LUN, + 110GB of snapshot space + 5GB for TWS snapshot = 215GB total  nbs.sparseTWS = 0 (Fully provisioned TWS, Fully Provisioned Source LUN) Filesystem size = 100GB Fully provisioned iSCSI LUN, + 110GB of snapshot space + 100GB TWS snapshot = 320GB total  Obviously, the best situation here is with virtually provisioned LUNs.
Hello,  I would like to create an iscsi lun of size 100 GB and want to create a snapshot. After the snapshot I need to mount this for backup. If I need to mount the iscsi then do I need to reserve additional space for this on the file syetm. I am planning to create the file syetm with 250 GB ( 100 GB Iscsi, 100 GB for the first snap and 50 % (50 Gb) for the rate of change of data) for this. Is this sufficient for satisfying the above condition? or Do I need to have more space for this.  Thanks Mohan
Iscsi Lun Sizing with Snapsure snapshot