Be aware that the commit log will block and not flush for commitlog_sync_batch_window_in_ms 

Aaron Morton
Freelance Developer

On 3/02/2012, at 5:44 AM, A J wrote:

Hello, when you set 'commitlog_sync: batch' on all the nodes in a
multi-DC cluster and call writes with CL=ALL, does the operation wait
till the write is flushed to all the disks on all the nodes ?