[PATCH 1/6] fs: add hole punching to fallocate

[Ted Ts'o]

On Tue, Nov 09, 2010 at 03:42:42PM +1100, Dave Chinner wrote:
> Implementation is up to the filesystem. However, XFS does (b)
> because:
> 
> 	1) it was extremely simple to implement (one of the
> 	   advantages of having an exceedingly complex allocation
> 	   interface to begin with :P)
> 	2) conversion is atomic, fast and reliable
> 	3) it is independent of the underlying storage; and
> 	4) reads of unwritten extents operate at memory speed,
> 	   not disk speed.

Yeah, I was thinking that using a device-style TRIM might be better
since future attempts to write to it won't require a separate seek to
modify the extent tree.  But yeah, there are a bunch of advantages of
simply mutating the extent tree.

While we're on the subject of changes to fallocate, what do people
think of FALLOC_FL_EXPOSE_OLD_DATA, which requires either root
privileges or (if capabilities are in use) CAP_DAC_OVERRIDE &&
CAP_MAC_OVERRIDE && CAP_SYS_ADMIN.  This would allow a trusted process
to fallocate blocks with the extent already marked initialized.  I've
had two requests for such functionality for ext4 already.  

(Take for example a trusted cluster filesystem backend that checks the
object checksum before returning any data to the user; and if the
check fails the cluster file system will try to use some other replica
stored on some other server.)

    		     	  		    	 - Ted