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

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Message-ID: <Pine.LNX.4.64.0708121901290.28963@fbirervta.pbzchgretzou.qr>
Date:	Sun, 12 Aug 2007 19:03:44 +0200 (CEST)
From:	Jan Engelhardt <jengelh@...putergmbh.de>
To:	david@...g.hm
cc:	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 Aug 12 2007 09:39, david@...g.hm wrote:
>
> now, I am not an expert on either option, but three are a couple things that I
> would question about the DRDB+MD option
>
> 1. when the remote machine is down, how does MD deal with it for reads and
> writes?

I suppose it kicks the drive and you'd have to re-add it by hand unless done by
a cronjob.

> 2. MD over local drive will alternate reads between mirrors (or so I've been
> told), doing so over the network is wrong.

Certainly. In which case you set "write_mostly" (or even write_only, not sure
of its name) on the raid component that is nbd.

> 3. when writing, will MD wait for the network I/O to get the data saved on the
> backup before returning from the syscall? or can it sync the data out lazily

Can't answer this one - ask Neil :)




	Jan
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