Are SequenceFiles split? If so, how?

[Barnet Wagman]

Suppose a SequenceFile (containing keys and values that are 
BytesWritable) is used as input. Will it be divided into InputSplits?  
If so, what's the criteria use for splitting?

I'm interested in this because I need to control the number of map tasks 
used, which (if I understand it correctly), is equal to the number of 
InputSplits.

thanks,

bw