vfs.write_behind

The handbook says:

	The vfs.write_behind sysctl variable defaults to 1 (on).
	This tells the file system to issue media writes as full
	clusters are collected, which typically occurs when
	writing large sequential files. The idea is to avoid
	saturating the buffer cache with dirty buffers when it
	would not benefit I/O performance. However, this may
	stall processes and under certain circumstances you may
	wish to turn it off.

Looking through the documentation, I can't find an
explaination
of what a cluster is.  Multiple blocks?

How would this stall processes?  Seems backwards.  If you
don't
have write-behind, a process would block until the data
gets
written to the media.

So if vfs.write_behind is 0, then a larger number of
smaller
writes are issued?
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