svn commit: r217828 - projects/graid/head/sys/geom/raid

[Robert Watson]

On Tue, 25 Jan 2011, John Baldwin wrote:

> Hmm, so this allocates bio's to make the dump work.  I believer other dump 
> routines in other drivers do not do this, but instead use pre-allocated 
> commands to schedule dump I/O requests.  Would it be possible to 
> pre-allocate the bio that is used here when dumping is enabled and reuse it 
> for each g_raid_dump() call without free'ing it when the I/O is finished?

Yeah, same concern here: memory allocation and freeing in the dump path leads 
to less robustness.  Part of why the existing geom/dump interaction was so 
simplistic was to avoid exactly this problem by not needing lots of bio 
allocaiton/replication/etc on the way up/down the stack.  Likewise, the goal 
was to minimise dependence on correct operation of the kernel scheduler.  Some 
storage device drivers are poorly behaved in this regard, but introucing new 
dependencies in the stack just makes it harder to fix those drivers.

(A similar discussion is taking place in the context of network dumping, where 
it's really important not to introduce further large kernel subsystem 
dependencies: you want debugging paths to depend on as little in the way of 
sensitive global data structures, locks, etc, as possible.)

Robert