From SYMCLI, you can check the state of the pool.  Here's a pool I just added two devices to:  C:\>symcfg -sid 1234 show -pool FC_R5_15K -thin  Symmetrix ID: 00076541234  Symmetrix ID                  : 00076541234 Pool Name                     : FC_R5_15K Pool Type                     : Thin Disk Location                 : Internal Technology                    : FC Dev Emulation                 : FBA Dev Configuration             : RAID-5(3+1) Pool State                    : Balancing # of Devices in Pool          :  8 # of Enabled Devices in Pool  :  8  Enabled Devices(8):   {    -----------------------------------------------------------------    Sym         Usable       Free       Used Full FLG Device    Dev         Tracks     Tracks     Tracks  (%)  S  State    -----------------------------------------------------------------    1666       1078785     994509      84276    7  .  Enabled    1667       1078785     993117      85668    7  .  Enabled    1668       1078785     993069      85716    7  .  Enabled    1669       1078785     996585      82200    7  .  Enabled    166A       1078785     994833      83952    7  .  Enabled    166B       1078785     993057      85728    7  .  Enabled    166C       1078645     993645      140       1  .  Enabled    166D       1078645     993645      140       1  .  Enabled  Note that 166C and 166D are the new members.   And yes, from the SMC Gui Help file:  Start Write Balancing — when modifying a thin pool, this specifies to start balancing I/O across all the DATA devices in the pool after the new devices are added. This feature is only supported on Symmetrix arrays running Enginuity 5874.207.166 or higher.  A rebalance of a large pool can take quite a long time.  You can increase the number of devices that are rebalanced at a time, but you might eventually incur a bit of perfomance hit.  Let us know if this helps!
Hi,  I added TDATs in a Thin pool a few days ago and started a Pool rebalancing. Now after a few days the new TDATs are still showing only 2% usage, the old ones are showing 78%. How can I verify if the balancing is still in progress?  Also is the GUI "Start Write Balancing..." option on a thin pool the same as the SYMCLI command: symconfigure -sid XXX -cmd "start balancing on pool= type=thin;" commit –nop ?  Thanks
VMAX Thin Pool rebalancing State?