netdev - Re: [RFD] Layering: Use-Case Composers (was: DRBD - what is it, anyways? [compare with e.g. NBD + MD raid])

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Date:	Mon, 13 Aug 2007 01:31:24 -0700 (PDT)
From:	david@...g.hm
To:	David Greaves <david@...eaves.com>
cc:	Paul Clements <paul.clements@...eleye.com>,
	Jan Engelhardt <jengelh@...putergmbh.de>,
	Al Boldi <a1426z@...ab.com>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, netdev@...r.kernel.org,
	linux-raid@...r.kernel.org
Subject: Re: [RFD] Layering: Use-Case Composers (was: DRBD - what is it,
 anyways? [compare with e.g. NBD + MD raid])

On Mon, 13 Aug 2007, David Greaves wrote:

> david@...g.hm wrote:
>>  per the message below MD (or DM) would need to be modified to work
>>  reasonably well with one of the disk components being over an unreliable
>>  link (like a network link)
>>
>>  are the MD/DM maintainers interested in extending their code in this
>>  direction? or would they prefer to keep it simpler by being able to
>>  continue to assume that the raid components are connected over a highly
>>  reliable connection?
>>
>>  if they are interested in adding (and maintaining) this functionality then
>>  there is a real possibility that NBD+MD/DM could eliminate the need for
>>  DRDB. however if they are not interested in adding all the code to deal
>>  with the network type issues, then the argument that DRDB should not be
>>  merged becouse you can do the same thing with MD/DM + NBD is invalid and
>>  can be dropped/ignored
>>
>>  David Lang
>
> As a user I'd like to see md/nbd be extended to cope with unreliable links.
> I think md could be better in handling link exceptions. My unreliable memory 
> recalls sporadic issues with hot-plug leaving md hanging and certain lower 
> level errors (or even very high latency) causing unsatisfactory behaviour in 
> what is supposed to be a fault 'tolerant' subsystem.
>
>
> Would this just be relevant to network devices or would it improve support 
> for jostled usb and sata hot-plugging I wonder?

good question, I suspect that some of the error handling would be similar 
(for devices that are unreachable not haning the system for example), but 
a lot of the rest would be different (do you really want to try to 
auto-resync to a drive that you _think_ just reappeared, what if it's a 
different drive? how can you be sure?) the error rate of a network is gong 
to be significantly higher then for USB or SATA drives (although I suppose 
iscsi would be limilar)

David Lang
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