Message-ID: <alpine.DEB.2.00.1312062154530.1560@cobra.newdream.net>
Date:	Fri, 6 Dec 2013 21:57:20 -0800 (PST)
From:	Sage Weil <sage@...tank.com>
To:	Mark Doffman <mark.doffman@...ethink.co.uk>
cc:	ceph-devel@...r.kernel.org,
	Rob Taylor <rob.taylor@...ethink.co.uk>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-nfs@...r.kernel.org
Subject: Re: [PATCH 2/4] Documentation: Document the cephroot functionality

On Wed, 20 Nov 2013, mark.doffman@...ethink.co.uk wrote:
> From: Rob Taylor <rob.taylor@...ethink.co.uk>
> 
> Document using the cephfs as a root device, its purpose,
> functionality and use.
> 
> Signed-off-by: Mark Doffman <mark.doffman@...ethink.co.uk>
> Signed-off-by: Rob Taylor <rob.taylor@...ethink.co.uk>
> Reviewed-by: Ian Molton <ian.molton@...ethink.co.uk>
> ---
>  Documentation/filesystems/{ => ceph}/ceph.txt |  0
>  Documentation/filesystems/ceph/cephroot.txt   | 81 +++++++++++++++++++++++++++
>  2 files changed, 81 insertions(+)
>  rename Documentation/filesystems/{ => ceph}/ceph.txt (100%)
>  create mode 100644 Documentation/filesystems/ceph/cephroot.txt
> 
> diff --git a/Documentation/filesystems/ceph.txt b/Documentation/filesystems/ceph/ceph.txt
> similarity index 100%
> rename from Documentation/filesystems/ceph.txt
> rename to Documentation/filesystems/ceph/ceph.txt
> diff --git a/Documentation/filesystems/ceph/cephroot.txt b/Documentation/filesystems/ceph/cephroot.txt
> new file mode 100644
> index 0000000..ae0f5bb
> --- /dev/null
> +++ b/Documentation/filesystems/ceph/cephroot.txt
> @@ -0,0 +1,81 @@
> +Mounting the root filesystem via Ceph (cephroot)
> +===============================================
> +
> +Written 2013 by Rob Taylor <rob.taylor@...ethink.co.uk>
> +
> +derived from nfsroot.txt:
> +
> +Written 1996 by Gero Kuhlmann <gero@...inix.han.de>
> +Updated 1997 by Martin Mares <mj@...ey.karlin.mff.cuni.cz>
> +Updated 2006 by Nico Schottelius <nico-kernel-nfsroot@...ottelius.org>
> +Updated 2006 by Horms <horms@...ge.net.au>
> +
> +
> +
> +In order to use a diskless system, such as an X-terminal or printer server
> +for example, it is necessary for the root filesystem to be present on a
> +non-disk device. This may be an initramfs (see Documentation/filesystems/
> +ramfs-rootfs-initramfs.txt), a ramdisk (see Documentation/initrd.txt), a
> +filesystem mounted via NFS or a filesystem mounted via Ceph. The following
> +text describes on how to use Ceph for the root filesystem.
> +
> +For the rest of this text 'client' means the diskless system, and 'server'
> +means the Ceph server.
> +
> +
> +1.) Enabling cephroot capabilities
> +    -----------------------------
> +
> +In order to use cephroot, CEPH_FS needs to be selected as
> +built-in during configuration. Once this has been selected, the cephroot
> +option will become available, which should also be selected.
> +
> +In the networking options, kernel level autoconfiguration can be selected,
> +along with the types of autoconfiguration to support. Selecting all of
> +DHCP, BOOTP and RARP is safe.
> +
> +
> +2.) Kernel command line
> +    -------------------
> +
> +When the kernel has been loaded by a boot loader (see below) it needs to be
> +told what root fs device to use. And in the case of cephroot, where to find
> +both the server and the name of the directory on the server to mount as root.
> +This can be established using the following kernel command line parameters:
> +
> +root=/dev/ceph
> +
> +This is necessary to enable the pseudo-Ceph-device. Note that it's not a
> +real device but just a synonym to tell the kernel to use Ceph instead of
> +a real device.
> +
> +cephroot=<monaddr>:/[<subdir>],<ceph-opts>
> +
> +  <monaddr>     Monitor address. Each takes the form host[:port]. If the port
> +		is not specified, the Ceph default of 6789 is assumed.
> +
> +  <subdir>	A subdirectory subdir may be specified if a subset of the file
> +		system is to be mounted
> +
> +  <ceph-opts>	Standard Ceph options. All options are separated by commas.
> +		See Documentation/filesystems/ceph/ceph.txt for options and
> +		their defaults.

Maybe there is an existing convention here, but: it seems like it would be 
simpler to do something like

 cephroot=<ip[:<port>][,...]>:/[<subdir>]

i.e., the existing syntax used by mount, that (among other things) can 
also include a port, or be a list of mon ips, so that the parsing code 
can be re-used.  Then,

 cephopts=<ceph-opts>

Hopefully this would avoid the parsing in root.c and make things behave 
more consistently with respect to how mount(8) is used?

sage

> +
> +4.) References
> +    ----------
> +
> +
> +5.) Credits
> +    -------
> +
> +  cephroot was derived from nfsroot by Rob Taylor <rob.taylor@...ethink.co.uk>
> +  and Mark Doffman <mark.doffman@...ethink.co.uk>
> +
> +  The nfsroot code in the kernel and the RARP support have been written
> +  by Gero Kuhlmann <gero@...inix.han.de>.
> +
> +  The rest of the IP layer autoconfiguration code has been written
> +  by Martin Mares <mj@...ey.karlin.mff.cuni.cz>.
> +
> +  In order to write the initial version of nfsroot I would like to thank
> +  Jens-Uwe Mager <jum@...bis.han.de> for his help.
> -- 
> 1.8.4
> 
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