linux-kernel - Re: [Nbd] [RFC 4/4] nbd: Add support for nbd as root device

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Message-ID: <20150130080400.GD16879@pengutronix.de>
Date:	Fri, 30 Jan 2015 09:04:00 +0100
From:	Markus Pargmann <mpa@...gutronix.de>
To:	Wouter Verhelst <w@...r.be>
Cc:	Paul Clements <Paul.Clements@...eleye.com>,
	nbd-general@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
	kernel@...gutronix.de
Subject: Re: [Nbd] [RFC 4/4] nbd: Add support for nbd as root device

Hi,

On Fri, Jan 30, 2015 at 12:42:54AM +0100, Wouter Verhelst wrote:
> On Tue, Jan 13, 2015 at 02:44:25PM +0100, Markus Pargmann wrote:
> > Adding support to nbd to use it as a root device. This code essentially
> > provides a minimal nbd-client implementation within the kernel. It opens
> > a socket and makes the negotiation with the server. Afterwards it passes
> > the socket to the normal nbd-code to handle the connection.
> > 
> > The arguments for the server are passed via module parameter. The
> > module parameter has the format
> > '[<SERVER_IP>:]<SERVER_PORT>/<EXPORT_NAME>'.
> > SERVER_IP is optional. If it is not available it will use the
> > root_server_addr transmitted through DHCP.
> > 
> > Based on those arguments, the connection to the server is established
> > and is connected to the nbd0 device. The rootdevice therefore is
> > root=/dev/nbd0.
> 
> Not that I'm opposed to this, but you do realize that doing nbd-client
> from initramfs or similar is possible, right? Most initramfs
> implementations these days support it.

Yes, that was the first idea how to implement a complete netboot for an
embedded ARM device. However, an initramfs is at least around 1MB in
size which has to be loaded using tftp. As the essential nbd-client
connection setup and negotiation is quite small I decided to go with
nbd-root support.

Also it is quite useful to have nbd-root support much like nfsroot
directly built-in for debugging purposes. It has the big advantage of
booting/testing read-only filesystem images for embedded systems without
the need for an initramfs.

Best regards,

Markus

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