Re: [dm-devel] NetBSD libdevmapper port

[Brett Lymn <blymn%baesystems.com.au@localhost>]

Hi - I am mentoring Adam in this project

On Fri, Jun 13, 2008 at 12:51:48PM -0400, Mikulas Patocka wrote:
> 
> If you are rewriting it --- have you somehow thought about avoiding 
> suspend?
>

I assume you are talking about handling a suspend of a device
underlying the LVM.  At the moment, NetBSD does not have the facility
to suspend a device in this manner so I dont think we will have this
issue (yet).
 
> 
> A Linux LVM does something like: suspend old table, write to disk with 
> direct i/o, resume new table. I'd suggest that you invent some method how 
> to batch these operations into single syscall

I think this should be doable with the API that Adam is using - in
NetBSD there is a thing called proplib which is a method of passing a
very limited form of XML into the kernel - Adam is using proplib to
pass the lvm parameters into the kernel.

>  --- the question --- how to do it portably on all 
> NetBSD architectures?), 

Actually, most of the operations you have listed are, from memory,
machine independent code - there is very little of the kernel that
actually is architecture specific we work hard to keep it that way.

> 
> --- if you port lvm2 as it is, you'll have to audit (and maybe rewrite) 
> many parts of NetBSD kernel for not waiting for I/O. If you do it badly, 
> you'll get deadlocks.
> 

The head of the bleeding edge NetBSD code (netbsd-current) is having a
lot of work done on it to make the kernel re-entrant and
multi-threaded.  This may be a bit of a bonus in terms of what you are
saying because there should be locks that need to be held to perform
the i/o - it may be a case of just making the acquiring of these locks
non-blocking (or it may not be an issue at all).  We shall need to
wait and see on that one.

Thanks for the input.

-- 
Brett Lymn
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