kern/145999: optional offset for `mdconfig -t vnode'

[Mikhail T.]

>Number:         145999
>Category:       kern
>Synopsis:       optional offset for `mdconfig -t vnode'
>Confidential:   no
>Severity:       non-critical
>Priority:       medium
>Responsible:    freebsd-bugs
>State:          open
>Quarter:        
>Keywords:       
>Date-Required:
>Class:          change-request
>Submitter-Id:   current-users
>Arrival-Date:   Fri Apr 23 19:00:12 UTC 2010
>Closed-Date:
>Last-Modified:
>Originator:     Mikhail T.
>Release:        FreeBSD 7.3-STABLE amd64
>Organization:
Virtual Estates, Inc.	http://sybpipe.com/
>Environment:
System: FreeBSD aldan.narawntapu 7.3-STABLE FreeBSD 7.3-STABLE #3: Fri Apr 9 17:20:45 EDT 2010 mi at aldan.narawntapu:/var/src/sys/amd64/compile/SILVER-SMP amd64

>Description:

	It is already possible to use only part of the file by
	explicitly specifying the size (mdconfig -s) of the md.

	For completeness, it should also be possible to request, that
	certain number of bytes be skipped from the beginning...

	If necessary, requiring that this offset be a multiple of
	512 or some such is acceptable.

	For example:

		mdconfig -t vnode -f zhoo -b 8192

	(-b would stand for "begin").

>How-To-Repeat:

	This could be useful for people dealing with files, that have
	file-system images embedded in them -- preceded by some other
	data...

	The only existing work-around for such situations is using
	something like dd skip= to produce a separate file, which
	can almost double the storage requirements and cause other
	inconvenience.

	I propose, that the md_base field of the md_ioctl struct
	(currently in use only in the MD_PRELOAD cases) be used
	for this purpose.

>Fix:

	The userland changes (add an option for mdconfig.c) is easy.
	The kernel changes required are beyond my knowledge...
>Release-Note:
>Audit-Trail:
>Unformatted: