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Date: Mon, 30 May 2011 19:22:55 +0100 (BST)
From: Robert Watson <rwatson@FreeBSD.org>
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To: Kostik Belousov <kostikbel@gmail.com>
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Cc: svn-src-head@freebsd.org, Rick Macklem <rmacklem@freebsd.org>,
	svn-src-all@freebsd.org, src-committers@freebsd.org
Subject: Re: svn commit: r222466 - head/sbin/umount
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On Mon, 30 May 2011, Kostik Belousov wrote:

> On Mon, May 30, 2011 at 01:48:53PM +0100, Robert Watson wrote:
>> On Sun, 29 May 2011, Rick Macklem wrote:
>>
>>> Modify the umount(8) command so that it doesn't do a sync(2) syscall 
>>> before unmount(2) for the "-f" case. This avoids a forced dismount from 
>>> getting stuck for an NFS mountpoint in sync() when the server is not 
>>> responsive. With this commit, forced dismounts should normally work for 
>>> the NFS clients, but can take up to about 1minute to complete.
>>
>> I'm actually a bit confused about why umount(8) calls sync(2) at all: 
>> surely it's the responsibility of the file system, rather than the userland 
>> tool, to ensure consistency subject to file system configuration and 
>> unmount-time flags?
> This call is from the same department as triple-sync before reboot, IMO.

No doubt. :-)

If the sync(2) has actual consistency and reliability benefits, it should 
probably be done by the umount(2) system call, so that other future 
auto-mounters, etc, also get the same result, rather than having to encode it 
in every application.  If it's done on blind faith, perhaps it shouldn't be 
done at all.

Robert