Bug 1206365 - Live Merge: Active layer merge is not properly synchronized with vdsm
Summary: Live Merge: Active layer merge is not properly synchronized with vdsm
Alias: None
Product: Red Hat Enterprise Linux 7
Classification: Red Hat
Component: libvirt
Version: 7.1
Hardware: Unspecified
OS: Unspecified
Target Milestone: rc
: 7.1
Assignee: Peter Krempa
QA Contact: Virtualization Bugs
Whiteboard: storage
Depends On:
Blocks: 1155583 1206355 1206722 1207808 1208021
TreeView+ depends on / blocked
Reported: 2015-03-26 21:26 UTC by Adam Litke
Modified: 2015-11-19 06:25 UTC (History)
18 users (show)

Fixed In Version: libvirt-1.2.14-1.el7
Doc Type: Bug Fix
Doc Text:
Cause: A recent bug fix introduced a regression when a block job is finished via the synchronous block job API. The code that is updating the backing chain of the disk after the block job finishes was extracted into a separate thread and thus allowed to race with other threads. Consequence: If one thread is executing the synchronous abort/pivot operation while a second thread is already waiting to access the domain definition to read the backing chain, the update operation will be queued after the query thread and thus the query thread will not get the updated backing chain data. Fix: When a synchronous Abort/Pivot operation is requested, the backing chain update is done in the same thread that is waiting for completion of the job. This assures that the backing chain info is updated in time. Result: Different threads are able to get correct backing chain data after a synchronous blockJobAbort is issued.
Clone Of: 1206355
: 1208021 (view as bug list)
Last Closed: 2015-11-19 06:25:02 UTC
Target Upstream Version:

Attachments (Terms of Use)

System ID Private Priority Status Summary Last Updated
Red Hat Bugzilla 1202719 0 high CLOSED Libvirtd was restarted when do active blockcommit while there is a blockpull job running 2021-02-22 00:41:40 UTC
Red Hat Product Errata RHBA-2015:2202 0 normal SHIPPED_LIVE libvirt bug fix and enhancement update 2015-11-19 08:17:58 UTC

Internal Links: 1202719

Description Adam Litke 2015-03-26 21:26:31 UTC
+++ This bug was initially created as a clone of Bug #1206355 +++

Description of problem:

After completing an active layer merge, vdsm's representation of the new volume chain might not be properly synchronized.  When this happens, engine will report a failure to delete the snapshot even though libvirt successfully merged it.

Version-Release number of selected component (if applicable):
libvirt-1.2.8-16.el7_1.2.x86_64 from http://download.devel.redhat.com/brewroot/packages/libvirt/1.2.8/16.el7_1.2/x86_64/ which is a candidate build for RHEL-7.1 zStream

vdsm-4.16.12-28.gitf03bb74.el7.x86_64 : 3.5 branch tip

How reproducible: Intermittent but easy to reproduce

Steps to Reproduce:
1. Create a VM with 3 BLOCK disks (1 thin and 2 preallocated)
2. Start the VM
3. Create a snapshot
4. Delete the snapshot

Actual results:
Snapshot may fail to delete on at least one disk

Expected results:
Snapshot removal is successful

Additional info:
The vdsm log reveals errors such as the following:

Thread-50::ERROR::2015-03-26 10:53:09,314::sampling::488::vm.Vm::(collect) vmId=`b3fbc637-9cc9-4b15-ba94-5a2ee1607785`::Stats function failed: <AdvancedStatsFunction _highWrite at 0x3133870>
Traceback (most recent call last):
  File "/usr/share/vdsm/virt/sampling.py", line 484, in collect
  File "/usr/share/vdsm/virt/sampling.py", line 359, in __call__
    retValue = self._function(*args, **kwargs)
  File "/usr/share/vdsm/virt/vm.py", line 292, in _highWrite
  File "/usr/share/vdsm/virt/vm.py", line 2537, in extendDrivesIfNeeded
    extend = [x for x in self._getExtendCandidates()
  File "/usr/share/vdsm/virt/vm.py", line 2489, in _getExtendCandidates
    capacity, alloc, physical = self._dom.blockInfo(drive.path, 0)
  File "/usr/share/vdsm/virt/vm.py", line 689, in f
    ret = attr(*args, **kwargs)
  File "/usr/lib/python2.7/site-packages/vdsm/libvirtconnection.py", line 111, in wrapper
    ret = f(*args, **kwargs)
  File "/usr/lib64/python2.7/site-packages/libvirt.py", line 646, in blockInfo
    if ret is None: raise libvirtError ('virDomainGetBlockInfo() failed', dom=self)
libvirtError: invalid argument: invalid path /rhev/data-center/dee92f86-2fef-4edf-ac51-135d12646bde/01f563fa-1aef-41ab-92d3-0d6561d4a731/images/31cfd267-e627-4cfe-8f33-f66b0db627c3/1863e178-c40a-42eb-803a-3d1f1ac6658a not assigned to domain

After talking to Eric Blake on IRC, it looks like another problem with libvirt where the domain XML is not updated before the virDomainBlockJobAbort API (used for pivoting an active layer merge) returns.  Therefore, our volume chain sync code gets an old value for the disk path which causes _highWrite to fail in this manner.

Comment 1 Adam Litke 2015-03-26 21:31:33 UTC
This is likely fallout from Bug 1202719 and the working theory is that there is a new race window...

A virDomainBlockCommit involving the active layer is completed (pivoted) by using a call to virDomainBlockJobAbort.  The Abort returns and the caller immediately retrieves the domain XML, but that call returns the XML before libvirt could update it to represent the new backing chain (post pivot).

Comment 2 Allon Mureinik 2015-03-27 05:01:23 UTC
This fix is required to support Live Merge in RHEVM.
I don't have the permissions to raise flags on this BZ, but I'd like to propose it for RHEL 7.1.z.

Comment 3 Shanzhi Yu 2015-03-27 06:47:15 UTC
well, I can reproduce it with rhevm3.5 env, although not 100 percent.
with packages



Comment 5 Peter Krempa 2015-03-30 09:31:04 UTC
Patches fixing the regression proposed upstream. Please note that upstream released the broken code only in the release candidate for the upcomming release so there should not be any upstream release with the flaw once this is fixed.


Comment 6 Peter Krempa 2015-03-31 07:35:11 UTC
Fixed upstream:

commit 630ee5ac6cf4e3be3f3e986897a289865dd2604b
Author: Peter Krempa <pkrempa>
Date:   Mon Mar 30 11:26:20 2015 +0200

    qemu: blockjob: Synchronously update backing chain in XML on ABORT/PIVOT
    When the synchronous pivot option is selected, libvirt would not update
    the backing chain until the job was exitted. Some applications then
    received invalid data as their job serialized first.
    This patch removes polling to wait for the ABORT/PIVOT job completion
    and replaces it with a condition. If a synchronous operation is
    requested the update of the XML is executed in the job of the caller of
    the synchronous request. Otherwise the monitor event callback uses a
    separate worker to update the backing chain with a new job.
    This is a regression since 1a92c719101e5bfa6fe2b78006ad04c7f075ea28
    When the ABORT job is finished synchronously you get the following call
     #0  qemuBlockJobEventProcess
     #1  qemuDomainBlockJobImpl
     #2  qemuDomainBlockJobAbort
     #3  virDomainBlockJobAbort
    While previously or while using the _ASYNC flag you'd get:
     #0  qemuBlockJobEventProcess
     #1  processBlockJobEvent
     #2  qemuProcessEventHandler
     #3  virThreadPoolWorker

commit 0c4474df4e10d27e27dbcda80b1f9cc14f4bdd8a
Author: Peter Krempa <pkrempa>
Date:   Mon Mar 30 11:26:19 2015 +0200

    qemu: Extract internals of processBlockJobEvent into a helper
    Later on I'll be adding a condition that will allow to synchronise a
    SYNC block job abort. The approach will require this code to be called
    from two different places so it has to be extracted into a helper.

commit 6b6c4ab8a6d2096bd5f50d2ae9b0a929fbaaf076
Author: Peter Krempa <pkrempa>
Date:   Mon Mar 30 11:26:18 2015 +0200

    qemu: processBlockJob: Don't unlock @vm twice
    Commit 1a92c719 moved code to handle block job events to a different
    function that is executed in a separate thread. The caller of
    processBlockJob handles locking and unlocking of @vm, so the we should
    not do it in the function itself.


Comment 14 Yang Yang 2015-07-03 06:24:36 UTC
I can register host with latest libvirt and vdsm to rhevm, so this one is not blocked by bug 1232665.

I can reproduce it with libvirt-1.2.13-1.el7.x86_64 and vdsm-4.16.21-1.el7ev.x86_64

Verified it with libvirt-1.2.17-1.el7 and vdsm-4.16.21-1.el7ev.x86_64

1. Prepare a running guest with 4 disks, (2 thin and 2 preallocated)
2. Create 4 snapshots: s4-->s3-->s2-->s1-->base
3. delete 3 snapshots: s4, s3, s1
4. create 2 snapshots: ss2-->ss1-->s2-->base
5. delete 1 snapshots: s2
6. create 2 snapshots: ss4-->ss3-->ss2-->ss1-->base
7. delete 4 snapshots: ss4, ss2, ss3, ss1

All ops work well so move it to verified.

Comment 16 errata-xmlrpc 2015-11-19 06:25:02 UTC
Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.


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