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From: Ed Maste <emaste@freebsd.org>
Date: Mon, 29 Jun 2015 15:40:57 -0400
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Subject: Re: svn commit: r284928 - head/usr.bin/ar
To: Adrian Chadd <adrian.chadd@gmail.com>
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> On 29 June 2015 at 06:48, Ed Maste <emaste@freebsd.org> wrote:
>> Author: emaste
>> Date: Mon Jun 29 13:48:44 2015
>> New Revision: 284928
>> URL: https://svnweb.freebsd.org/changeset/base/284928
>>
>> Log:
>>   speed up ar(1) on UFS file systems
>
> Is this potentially an issue for other applications? Why is it such a
> big issue for ar?

Yes, it could be an issue for other applications, and I'm all that
happy to have this workaround in ar(1); it's a quick workaround for
the issue at hand.

kib pointed me at the explanation in vfs_vnops.c which explains the
deadlock avoidance code well, although I'm still not sure why the
slowdown is so extreme.