UNIX for Advanced & Expert Users

I am trying to collect the sar output for around 90minutes.

When i do

sar 1 5000 >> /tmp/sar.out

It's not updating the sar.out file. When we decrease the 5000 to smaller number like 10, i can see the file sar.out updated after the 10seconds.If i kill my sar while it is running it's not putting any thing in sar.out file.
Is it normal behaviour of sar or should i see the increment in sar.out file after each execution interval.

I tried that, and I too got the same result. I think it would be due to buffered I/O, but I am not sure. I tried using 'sync' but there still wasn't any output in the file. Maybe the shell is doing some sort of internal buffering...