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

From: Richard W.M. Jones
Subject: Re: [Qemu-devel] [PATCH v5 00/27] block: Lock images when opening
Date: Tue, 24 May 2016 12:48:45 +0100
User-agent: Mutt/1.5.20 (2009-12-10)

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.


Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
libguestfs lets you edit virtual machines.  Supports shell scripting,
bindings from many languages.  http://libguestfs.org

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