To: Chuck Silvers <>
From: Juergen Hannken-Illjes <>
List: tech-kern
Date: 07/13/2004 22:04:53
On Sun, Jul 11, 2004 at 07:00:18PM -0700, Chuck Silvers wrote:
> On Mon, Jul 12, 2004 at 05:35:52AM +0900, YAMAMOTO Takashi wrote:
> > besides, i can't believe that lwp is a right place to put
> > a filesystem specific flag.
> I agree with this entirely.  this feature ought not require hacks like this.
> it seems to me that if the copy-on-write is going to use the same pool
> of buffers that the original fs is using, then the the COW ought to be
> resolved before modifying any data that the original fs is using.
> that would also avoid the recursion, but much more cleanly.

It is not that simple. When a COW is resolved it has to allocate a block
to save the unmodified block. This VOP_BALLOC() is the problem we are
talking about. It has to read the indirect blocks and allocate a free block
which may involve more lookups in cylinder groups etc.
If any of these reads (through bread()) needs a buffer AND getnewbuf() tries
to get a buffer by cleaning one from the queues this one may call the COW
again (and again ...).

> there are other ways of doing all this too.  it would be good if
> this could be discussed in public and some agreement reached.
> -Chuck

Jürgen Hannken-Illjes -