Module Name:	src
Committed By:	tron
Date:		Mon Aug 15 14:57:19 UTC 2005

Modified Files:
	src/sys/arch/xen/xen [netbsd-3]: xbdback.c

Log Message:
Pull up revision 1.12 (requested by tls in ticket #656):
Improvements to xbdback (the domain 0 driver that provides "xbd" virtual
disks to other domains) from Jed Davis, <jld@panix.com>:
	* Issue multiple requests when necessary rather than
	  assuming that arbitrary requests can be mapped into single
	  contiguous virtual address ranges.
	* Don't assume that all data for a request is consecutive
	  in memory.  With some client OSes, it's not.
	The above two changes fix data corruption issues with Linux
	clients with certain filesystem block sizes.
	* Gracefully handle memory or pool allocation failures after
	  beginning to handle a request from the ring.
	* Merge contiguous requests to avoid the "64K turns into 44K + 20K
	  and doubles the transactions per second at the disk" problem
	  caused by the 11-page limit caused by the structure of Xen
	  ring entries.  This causes a very slight performance decrease
	  for sequential 64K I/O if the disk is not already saturated with
	  requests (about 1%) but halves the transactions per second we
	  hit the disk with -- or better.  It even compensates for bizarre
	  Linux behaviour like breaking long requests up into 5.5K pieces.
	* Probably some stuff I forgot to mention.
Disk throughput (though not latency) is now much, much closer to the
"raw hardware" case than it was before.


To generate a diff of this commit:
cvs rdiff -r1.4.2.6 -r1.4.2.7 src/sys/arch/xen/xen/xbdback.c

Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.