Hi Kashyap Chamarthy:
thank you very much for answer my question:

一： lead to VM filesystem becoming read-only
1： test case
it  lead to VM filesystem becoming read-only test case as follows:

we want to snapshot for VM , to obtain VM incremental data，and use virsh blockcommit，qemu-img commit，qemu-img rebase  to shorten snapshot chain.
Details are as  follows（when VM  running state， we perform the following operations）:

(1)  the host machine control VM test
virsh snapshot-create-as mix snap1 --diskspec vda,file=/tmp/mul/loop-mix-commit-rebase/snap1-mix.img --disk-only --atomic --quiesce
virsh snapshot-create-as mix snap2 --diskspec vda,file=/tmp/mul/loop-mix-commit-rebase/snap2-mix.img --disk-only --atomic --quiesce
virsh snapshot-create-as mix snap3 --diskspec vda,file=/tmp/mul/loop-mix-commit-rebase/snap3-mix.img --disk-only --atomic --quiesce
virsh snapshot-create-as mix snap4 --diskspec vda,file=/tmp/mul/loop-mix-commit-rebase/snap4-mix.img --disk-only --atomic --quiesce
virsh blockcommit mix vda --base /tmp/mul/loop-mix-commit-rebase/snap1-mix.img --top /tmp/mul/loop-mix-commit-rebase/snap2-mix.img --wait --verbose
qemu-img commit -f qcow2 /tmp/mul/loop-mix-commit-rebase/snap1-mix.img
qemu-img rebase -f qcow2 /tmp/mul/loop-mix-commit-rebase/snap3-mix.img -b /tmp/mul/loop-mix-commit-rebase/mix.img -F qcow2 -p
virsh snapshot-delete mix snap1 --metadata
rm -fr /tmp/mul/loop-mix-commit-rebase/snap1-mix.img
virsh snapshot-delete mix snap2 --metadata
rm -fr /tmp/mul/loop-mix-commit-rebase/snap2-mix.img
virsh snapshot-create-as mix snap5 --diskspec vda,file=/tmp/mul/loop-mix-commit-rebase/snap5-mix.img --disk-only --atomic --quiesce
virsh snapshot-create-as mix snap6 --diskspec vda,file=/tmp/mul/loop-mix-commit-rebase/snap6-mix.img --disk-only --atomic --quiesce
virsh snapshot-create-as mix snap7 --diskspec vda,file=/tmp/mul/loop-mix-commit-rebase/snap7-mix.img --disk-only --atomic --quiesce
virsh snapshot-create-as mix snap8 --diskspec vda,file=/tmp/mul/loop-mix-commit-rebase/snap8-mix.img --disk-only --atomic --quiesce
virsh blockcommit mix vda --base /tmp/mul/loop-mix-commit-rebase/snap3-mix.img --top /tmp/mul/loop-mix-commit-rebase/snap6-mix.img --wait --verbose
qemu-img commit -f qcow2 /tmp/mul/loop-mix-commit-rebase/snap3-mix.img
qemu-img rebase -f qcow2 /tmp/mul/loop-mix-commit-rebase/snap7-mix.img -b /tmp/mul/loop-mix-commit-rebase/mix.img -F qcow2 -p
virsh snapshot-delete mix snap3 --metadata
rm -fr /tmp/mul/loop-mix-commit-rebase/snap3-mix.img
virsh snapshot-delete mix snap4 --metadata
rm -fr /tmp/mul/loop-mix-commit-rebase/snap4-mix.img
virsh snapshot-delete mix snap5 --metadata
rm -fr /tmp/mul/loop-mix-commit-rebase/snap5-mix.img
virsh snapshot-delete mix snap6 --metadata
rm -fr /tmp/mul/loop-mix-commit-rebase/snap6-mix.img
virsh snapshot-create-as mix snap9 --diskspec vda,file=/tmp/mul/loop-mix-commit-rebase/snap9-mix.img --disk-only --atomic --quiesce
virsh snapshot-create-as mix snap10 --diskspec vda,file=/tmp/mul/loop-mix-commit-rebase/snap10-mix.img --disk-only --atomic --quiesce
virsh snapshot-create-as mix snap11 --diskspec vda,file=/tmp/mul/loop-mix-commit-rebase/snap11-mix.img --disk-only --atomic --quiesce
virsh snapshot-create-as mix snap12 --diskspec vda,file=/tmp/mul/loop-mix-commit-rebase/snap12-mix.img --disk-only --atomic --quiesce
virsh blockcommit mix vda --base /tmp/mul/loop-mix-commit-rebase/snap7-mix.img --top /tmp/mul/loop-mix-commit-rebase/snap10-mix.img --wait --verbose
qemu-img commit -f qcow2 /tmp/mul/loop-mix-commit-rebase/snap7-mix.img
qemu-img rebase -f qcow2 /tmp/mul/loop-mix-commit-rebase/snap11-mix.img -b /tmp/mul/loop-mix-commit-rebase/mix.img -F qcow2 -p
virsh snapshot-delete mix snap7 --metadata
rm -fr /tmp/mul/loop-mix-commit-rebase/snap7-mix.img
virsh snapshot-delete mix snap8 --metadata
rm -fr /tmp/mul/loop-mix-commit-rebase/snap8-mix.img
virsh snapshot-delete mix snap9 --metadata
rm -fr /tmp/mul/loop-mix-commit-rebase/snap9-mix.img
virsh snapshot-delete mix snap10 --metadata
rm -fr /tmp/mul/loop-mix-commit-rebase/snap10-mix.img
virsh snapshot-create-as mix snap13 --diskspec vda,file=/tmp/mul/loop-mix-commit-rebase/snap13-mix.img --disk-only --atomic --quiesce
virsh snapshot-create-as mix snap14 --diskspec vda,file=/tmp/mul/loop-mix-commit-rebase/snap14-mix.img --disk-only --atomic --quiesce
virsh snapshot-create-as mix snap15 --diskspec vda,file=/tmp/mul/loop-mix-commit-rebase/snap15-mix.img --disk-only --atomic --quiesce
virsh snapshot-create-as mix snap16 --diskspec vda,file=/tmp/mul/loop-mix-commit-rebase/snap16-mix.img --disk-only --atomic --quiesce
virsh blockcommit mix vda --base /tmp/mul/loop-mix-commit-rebase/snap11-mix.img --top /tmp/mul/loop-mix-commit-rebase/snap14-mix.img --wait --verbose
qemu-img commit -f qcow2 /tmp/mul/loop-mix-commit-rebase/snap11-mix.img
qemu-img rebase -f qcow2 /tmp/mul/loop-mix-commit-rebase/snap15-mix.img -b /tmp/mul/loop-mix-commit-rebase/mix.img -F qcow2 -p
virsh snapshot-delete mix snap11 --metadata
rm -fr /tmp/mul/loop-mix-commit-rebase/snap11-mix.img
virsh snapshot-delete mix snap12 --metadata
rm -fr /tmp/mul/loop-mix-commit-rebase/snap12-mix.img
virsh snapshot-delete mix snap13 --metadata
rm -fr /tmp/mul/loop-mix-commit-rebase/snap13-mix.img
virsh snapshot-delete mix snap14 --metadata
rm -fr /tmp/mul/loop-mix-commit-rebase/snap14-mix.img
。。。。。。。。。。。。。。。。。。。。。。。。。。。。(the cycle test continue)

(2)  the VM  test
in the VM,  loop write data and  delete data operations。

2： test result
when the test run for a while(8 hour or 10 hour or 20 hour or 30 hour  and so on), the VM filesystem becoming read-only!
3： We are not sure the problem
      （1）when VM running, whether it can be used to modify snapshot's backingfile by qemu-img rebase(can only be used in vm offline？)? 
       （2）when VM running，whether it can be used to commit the snapshot's changes by qemu-img commit(can only be used in vm offline？)？
       

二： Run normal(ok) test
1： test case
      in order to eliminate potential problems，we only use virsh blockcommit shorten snapshot chain.
(1)  the host machine control VM test

virsh snapshot-create-as length snap1 --diskspec vda,file=/tmp/testsnapshot/image/snap1-length.img --disk-only --atomic --quiesce
virsh snapshot-create-as length snap2 --diskspec vda,file=/tmp/testsnapshot/image/snap2-length.img --disk-only --atomic --quiesce
virsh snapshot-create-as length snap3 --diskspec vda,file=/tmp/testsnapshot/image/snap3-length.img --disk-only --atomic --quiesce
virsh snapshot-create-as length snap4 --diskspec vda,file=/tmp/testsnapshot/image/snap4-length.img --disk-only --atomic --quiesce
virsh blockcommit length vda --base /tmp/testsnapshot/image/length.img --top /tmp/testsnapshot/image/snap3-length.img --wait --verbose
virsh snapshot-delete length snap1 --metadata
rm -fr /tmp/testsnapshot/image/snap1-length.img
virsh snapshot-delete length snap2 --metadata
rm -fr /tmp/testsnapshot/image/snap2-length.img
virsh snapshot-delete length snap3 --metadata
rm -fr /tmp/testsnapshot/image/snap3-length.img
virsh snapshot-create-as length snap5 --diskspec vda,file=/tmp/testsnapshot/image/snap5-length.img --disk-only --atomic --quiesce
virsh snapshot-create-as length snap6 --diskspec vda,file=/tmp/testsnapshot/image/snap6-length.img --disk-only --atomic --quiesce
virsh snapshot-create-as length snap7 --diskspec vda,file=/tmp/testsnapshot/image/snap7-length.img --disk-only --atomic --quiesce
virsh snapshot-create-as length snap8 --diskspec vda,file=/tmp/testsnapshot/image/snap8-length.img --disk-only --atomic --quiesce
virsh blockcommit length vda --base /tmp/testsnapshot/image/length.img --top /tmp/testsnapshot/image/snap7-length.img --wait --verbose
virsh snapshot-delete length snap4 --metadata
rm -fr /tmp/testsnapshot/image/snap4-length.img
virsh snapshot-delete length snap5 --metadata
rm -fr /tmp/testsnapshot/image/snap5-length.img
virsh snapshot-delete length snap6 --metadata
rm -fr /tmp/testsnapshot/image/snap6-length.img
virsh snapshot-delete length snap7 --metadata
rm -fr /tmp/testsnapshot/image/snap7-length.img
virsh snapshot-create-as length snap9 --diskspec vda,file=/tmp/testsnapshot/image/snap9-length.img --disk-only --atomic --quiesce
virsh snapshot-create-as length snap10 --diskspec vda,file=/tmp/testsnapshot/image/snap10-length.img --disk-only --atomic --quiesce
virsh snapshot-create-as length snap11 --diskspec vda,file=/tmp/testsnapshot/image/snap11-length.img --disk-only --atomic --quiesce
virsh snapshot-create-as length snap12 --diskspec vda,file=/tmp/testsnapshot/image/snap12-length.img --disk-only --atomic --quiesce
virsh blockcommit length vda --base /tmp/testsnapshot/image/length.img --top /tmp/testsnapshot/image/snap11-length.img --wait --verbose
virsh snapshot-delete length snap8 --metadata
rm -fr /tmp/testsnapshot/image/snap8-length.img
virsh snapshot-delete length snap9 --metadata
rm -fr /tmp/testsnapshot/image/snap9-length.img
virsh snapshot-delete length snap10 --metadata
rm -fr /tmp/testsnapshot/image/snap10-length.img
virsh snapshot-delete length snap11 --metadata
rm -fr /tmp/testsnapshot/image/snap11-length.img
virsh snapshot-create-as length snap13 --diskspec vda,file=/tmp/testsnapshot/image/snap13-length.img --disk-only --atomic --quiesce
virsh snapshot-create-as length snap14 --diskspec vda,file=/tmp/testsnapshot/image/snap14-length.img --disk-only --atomic --quiesce
virsh snapshot-create-as length snap15 --diskspec vda,file=/tmp/testsnapshot/image/snap15-length.img --disk-only --atomic --quiesce
virsh snapshot-create-as length snap16 --diskspec vda,file=/tmp/testsnapshot/image/snap16-length.img --disk-only --atomic --quiesce
virsh blockcommit length vda --base /tmp/testsnapshot/image/length.img --top /tmp/testsnapshot/image/snap15-length.img --wait --verbose
virsh snapshot-delete length snap12 --metadata
rm -fr /tmp/testsnapshot/image/snap12-length.img
virsh snapshot-delete length snap13 --metadata
rm -fr /tmp/testsnapshot/image/snap13-length.img
virsh snapshot-delete length snap14 --metadata
rm -fr /tmp/testsnapshot/image/snap14-length.img
virsh snapshot-delete length snap15 --metadata
rm -fr /tmp/testsnapshot/image/snap15-length.im

 (2)  the VM  test
in the VM,  loop write data and  delete data operations。

2： test result
the test run 200 hours， the test ok！！

Waiting for your reply, thank you very much!



justlibvirt@gmail.com
 
From: Kashyap Chamarthy
Date: 2015-10-13 21:55
To: justlibvirt@gmail.com
CC: libvir-list; libvirt-users; eblake
Subject: Re: [libvirt-users] virsh can't support VM offline blockcommit
On Tue, Oct 13, 2015 at 10:39:58AM +0800, justlibvirt@gmail.com wrote:
> Hi  everyone!
 
Hi,
 
Firstly, please don't post the same message multiple times in a row.  If
someone hasn't responded, they're either busy, or haven't noticed this
message.
 
>      I use the libvirt(version: 1.2.2) and QEMU(version: 2.2.1) to
>      test qemu snapshot features:
>    
>      I tried virsh blockcommit when VM offline,  the virsh blockcommit
>      failed: the error messase as below: error: Requested operation is
>      not valid: domain is not running when I start the VM,  the virsh
>      blockcommit  work fine!  my question is：we need virsh support
>      blockcommit when VM offline。how do？
 
As near as I know, 'blockcommit' hasn't yet gotten offline support.
 
>     we  test qemu snapshot as follows： we need  create  snapshot of
>     the cycle，and  by virsh blockcommit merge snapshot chain，but
>     when VM offline， the virsh blockcommit will fail， someone
>     suggest me：when VM offline，may use qemu-img commit to submit
>     snapshot turn，but qemu-img can't modify snapshot chain backing
>     relation，it need we use qemu-img  rebase amend snapshot‘s
>     backingfile，
 
As you note there's the 'qemu-img commit' primitive that allows you to
commit the changes in an overlay back into its base image:
 
If you have:  base.qcow2 <-- overlay1.qcow2 <-- overlay2.qcow2
 
And, if you do:
 
    $ qemu-img commit overlay2.qcow2
 
It'll commit the contents of overlay2.qcow2 into overlay1.qcow2.
 
And, indeed use the 'qemu-img rebase' to modify the backing file is the
only method I know of, for offline operations.
 
>     the main question is when we use qemu-img rebase
>     modify snapshot's backingfile, if someone start the VM, it  will
>     may lead to VM filesystem readonly。
 
Though I haven't tested this recently, why would it lead to VM
filesystem becoming read-only? 
 
>    we  should how to  solve the question?   thanks!  >
 
--
/kashyap