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Re: [Qemu-devel] [PATCH v5 00/27] block: Lock images when opening

From: Kevin Wolf
Subject: Re: [Qemu-devel] [PATCH v5 00/27] block: Lock images when opening
Date: Tue, 24 May 2016 14:46:15 +0200
User-agent: Mutt/1.5.21 (2010-09-15)

Am 24.05.2016 um 13:48 hat Richard W.M. Jones geschrieben:
> On Tue, May 17, 2016 at 03:35:09PM +0800, Fam Zheng wrote:
> > v5: - Change "lock-image=on/off" to "lock-mode=exclusive/shared/off".
> >       Default is "lock-mode=exclusive" to exclusively lock RW images and 
> > shared
> >       lock RO images; with lock-mode="shared", RW images are shared locked 
> > too;
> >       lock-mode=off turns off image locking completely.
> >     - Use F_OFD_SETLK fcntl so that close/dup on different fds are not a
> >       problem.
> >     - Update test cases.
> My comments after testing this patch set:
> * It's not possible to tell from the `qemu -help' output that this
>   binary supports the lock-mode option.  Please add this to the -help
>   output (under `-drive') so we can detect it in qemu.
> * I patched libguestfs to add the `lock-image=off' flag when the drive
>   is added readonly.  This permits libguestfs to read live guests.  I
>   also checked that writing to live guests is now forbidden, and it
>   is, which is good.  In the write-to-live-guest case libguestfs will
>   now fail with:
>   qemu-system-x86_64: -drive 
> file=/var/tmp/centos-6.img,cache=writeback,id=hd0,if=none: Failed to lock 
> image
> So definitely we need this option to be reflected in the -help output.

While you are right that maybe we should mentioned the new options in
-help for human users, help texts are not meant to be parsed. If you do
parse them and a changed help text breaks your code in the future,
that's your problem.

Consider using query-qmp-schema instead, that one is actually meant to
be processed by machines.


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