dd of mounted filesystem

[Dru]

On Thu, 11 Dec 2003, Dan Nelson wrote:

> 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.

You mean, run "makesnap_ffs" first? I've been meaning to play with that
one, I'll have to try it out.

Dru