Re: [Nbd] distributed, fault-tolerant network block device

[Wouter Verhelst <w@...112...>]

On Sat, Mar 31, 2012 at 10:43:02PM +0200, Corin Langosch wrote:
> Hi,
> 
> I'm looking for a distributed, fault-tolerant network storage system which
> exposes block devices (not filesystems) on the clients. Matching my
> requirements I only found ceph's rdb, but it's still very experimental as far
> as I know. So I'm thinking about implementing the system myself using nbd and
> the design of moosefs, http://www.moosefs.org/, which is quite simple (the
> single point of failure master server is ok for me).
> 
> But there's a question left: I read nbd can easily be deadlocked when not
> properly dealt with memory requests:
>  
> 
>     when the system is short of memory, it tries to write back dirty pages. So
>     the nbd client asks the nbd server to write back data, but as nbd-server is
>     a userland process, it may require creating dirty pages to fullfill the
>     request. 

You can only hit that specific deadlock if you run nbd-client and
nbd-server on the same machine. If you don't, there's still a chance for
another deadlock (if you put your swapspace on the NBD device), but the
chance of that happening is significantly lower.

-- 
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