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[Qemu-devel] [Bug 1721788] Re: Failed to get shared "write" lock with 'q


From: Daniel Berrange
Subject: [Qemu-devel] [Bug 1721788] Re: Failed to get shared "write" lock with 'qemu-img info'
Date: Fri, 06 Oct 2017 14:28:37 -0000

The QEMU process that has the disk image open may be doing I/O that
causes qcow2 metadata to be changed. Such changes could confuse the
'qemu-img' process it is was reading the metadata at the same time it
was being changed, causing bad data to be printed or worse. So requiring
a write lock is the 'safe' thing to do for reliability in the worst
case. You can override this by passing '--force-share' arg though.

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https://bugs.launchpad.net/bugs/1721788

Title:
  Failed to get shared "write" lock with 'qemu-img info'

Status in QEMU:
  New

Bug description:
  When running 'qemu-img info test.qcow2' while test.qcow2 is currently
  used by a Qemu process, I get the error

  qemu-img: Could not open 'test.qcow2': Failed to get shared "write"
  lock.

  
  Why does displaying information about a disk image need a write lock for the 
file?

  Steps to reproduce:

  qemu-img create -f qcow2 test.qcow2 10M
  qemu-system-x86_64 -nographic -drive file=test.qcow2
  qemu-img info test.qcow2

  The above was tested with Qemu version 2.10.0

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