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bug#10349: tail: fix --follow on FhGFS remote file systems

From: Sven Breuner
Subject: bug#10349: tail: fix --follow on FhGFS remote file systems
Date: Fri, 23 Dec 2011 00:49:02 +0100
User-agent: Mozilla/5.0 (X11; Linux i686 on x86_64; rv:8.0) Gecko/20111105 Thunderbird/8.0

Hi Alan,

Alan Curry wrote on 12/22/2011 10:50 PM:
Bob Proulx writes:

Jim Meyering wrote:
Are there so many new remote file systems coming into use now?
That are not listed in /usr/include/linux/magic.h?

The past can always be enumerated.  The future is always changing.  It
isn't possible to have a complete list of future items.  It is only
possible to have a complete list of past items.  The future is not yet

Between past and future is the present, i.e. the currently running kernel.
Shouldn't it return an error when you use an interface that isn't implemented
by the underlying filesystem? Why doesn't this happen?

Actually, the kernel provides a nice generic inotify framework for file systems, so that inotify support on a single machine comes almost automatically. In the case of FhGFS (and I assume it's similar for other network file systems) inotify works perfectly if the process modifying a file and the process inotify-watching the file are running on the same machine. Since inotify is generally a good thing and nice to have, you wouldn't want to do anything to make it return an error here.

However, specifically the tail utility is also often used in a cluster environment when some compute nodes are writing a job log file and you are on a different machine watching that log file with "tail -f". Supporting this distributed inotify use case would mean quite some extra work for file systems and would rarely be useful for other distributed use cases (as distributed applications can normally just communicate directly via messages if they want to notify another machine about file updates, thus they won't need anything like inotify), which is probably the reason why most network file systems don't support distributed inotify.

(For FhGFS, we're actually considering to add distributed inotify support, but since it's a low priority task for the reasons mentioned above, we probably won't add it in the near future.)

Best regards,

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