dd of mounted filesystem

[Dan Nelson]

In the last episode (Dec 11), Matthew Seaman said:
> On Thu, Dec 11, 2003 at 02:54:12PM -0500, Dru wrote:
> > Can anyone describe or point me to resources explaining why it is
> > dangerous to dd a filesystem while it is mounted? Is it still
> > considered to be dangerous if the system is first dropped down to
> > single-user mode?
> 
> Remember that dd(1) traverses the block device sequentially, but that
> most FS accesses are random, so any particular change can span either
> side of dd(1)'s offset.  Also that dd'ing from the block device
> bypasses the usual machinery for doing file IO -- machinery that is
> designed under the premise that it will have sole control over what
> gets read or written where and when.

On current you can get around the consistency problem by dd'ing a
snapshot of the filesystem, just like dump's -L flag does.

-- 
	Dan Nelson
	dnelson at allantgroup.com