linux-kernel - Re: name_to_handle_at() and persistent filesystem IDs

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Message-ID: <CAKgNAkhBxCZuvyofdXwRD2TWrXEO47O_873pO8WqUBkqY1zG1w@mail.gmail.com>
Date:	Mon, 17 Mar 2014 17:16:48 +0100
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Cc:	Linux-Fsdevel <linux-fsdevel@...r.kernel.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Andreas Dilger <adilger@...ger.ca>, NeilBrown <neilb@...e.de>,
	Miklos Szeredi <miklos@...redi.hu>,
	Bruce Fields <bfields@...ldses.org>,
	linux-man <linux-man@...r.kernel.org>,
	Christoph Hellwig <hch@...radead.org>,
	Ganesha NFS List <nfs-ganesha-devel@...ts.sourceforge.net>
Subject: Re: name_to_handle_at() and persistent filesystem IDs

Hello Aneesh,

On Sun, Mar 16, 2014 at 4:08 PM, Aneesh Kumar K.V
<aneesh.kumar@...ux.vnet.ibm.com> wrote:
> Michael Kerrisk <mtk.manpages@...il.com> writes:
>
>> Hello Aneesh,
>>
>> I'm currently working on a man page for name_to_handle_at() and
>> open_by_handle_at(), and I have a question relating to a point that
>> probably needs to be covered in the man page.
>>
>> Back in July 2010, in this thread:
>> http://thread.gmane.org/gmane.linux.file-systems/41782/focus=43131
>> you said:
>>
>> [[
>> mount id should not be looked at as a persistent identifier. It should
>> be used to derive a persistent identifier from /proc/self/mountinfo. The
>> persistent identifier could be the combination of device properties,
>> file system properties or the uuid which is going to be an optional
>> tag in /proc/self/mountinfo.
>> ]]
>>
>> In the man page, I'd like to briefly describe how one derives such a
>> persistent ID from mountinfo. AFAICS, the optional UUID tag in
>> /proc/self/mountinfo has not come to pass. So, what then is the
>> recommended practice for deriving the persistent ID?
>>
>
> Anything that work for the application. mount_id will indicate which
> mount it is ie. from /proc/self/mountinfo
>
> 30 20 8:1 / /boot rw,relatime - ext4 /dev/sda1 rw,data=ordered
>
> The value 30 helps us to figure out that we are looking at device
> /dev/sda1. With that we can derive the uuid using libblkid.  I am not sure we
> concluded anything really about how to identify an nfs mount. May be we
> can do that based on server and mount point details ?.But from the
> syscall point, what we return is mount_id, which gives a hint regarding
> which mount point we are talking about in /proc/self/mountinfo. From
> that information applications can use any method that work for them
> to derivce an unique identifier.
>
> I know that NFS ganesha use these syscall. May we can check with them
> what worked for them ? (Added to CC:)

Thanks, that helped, as as I hope is evident from the page I just sent.

Cheers,

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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