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Message-ID: <Pine.LNX.4.64.0708130127260.22470@asgard.lang.hm>
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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